Introduce compatibility with V8 7.3 (#138)

* Mechanical removal of use of non-maybe String::ToObject()

Due to the non-maybe version of String::ToObject() being deprecated and
altogether removed from V8 [1] it is necessary to migrate to using the maybe
version.

This commit is a mechanical change that uses the context at hand when calling
String::ToObject() to pass it to it.

The resulting MaybeLocal is then unwrapped with MaybeLocal::ToLocalChecked() as
I consider the verifications performed on the String instances to be sufficient
to ensure no crashes.

[1] https://chromium-review.googlesource.com/c/v8/v8/+/1172350/
    https://chromium.googlesource.com/v8/v8/+/c8376b0069ebe16c67acf90c3cda3457ddccba4f

* Mechanical removal of use of non-maybe Local<T>::ToString()

Due to the non-maybe version of Local<T>::ToString() being deprecated and
altogether removed from V8 [1] it is necessary to migrate to using the maybe
version.

This commit is a mechanical change that uses the context at hand when calling
Local<T>::ToString() to pass it to it.

The resulting MaybeLocal is then unwrapped with MaybeLocal::ToLocalChecked() as
I consider the context of the uses to be sufficiently safe.

[1] https://chromium-review.googlesource.com/c/v8/v8/+/1172350/
    https://chromium.googlesource.com/v8/v8/+/c8376b0069ebe16c67acf90c3cda3457ddccba4f

* Mechanical removal of the use of non-maybe Value::Int32Value() and NumberValue()

Due to the non-maybe version of Value::Int32Value() and Value::NumberValue()
being deprecated and altogether removed from V8 [1] it is necessary to migrate
to using the maybe versions.

This commit is a mechanical change that uses the context at hand when calling
Value::Int32Value() or Value::NumberValue() to pass it to the respective
function.

The resulting Maybe is then unwrapped with Maybe::ToChecked() as I consider the
verifications performed on the Value instances to be sufficient to ensure no
crashes.

[1] https://chromium-review.googlesource.com/c/v8/v8/+/1172350/
    https://chromium.googlesource.com/v8/v8/+/c8376b0069ebe16c67acf90c3cda3457ddccba4f

* Mechanical removal of use of String::Utf8Length() without isolate

Due to the version of String::Utf8Length() with no paramters being deprecated
and altogether removed from V8 [1] it is necessary to migrate to using the
version that accepts isolate as a parameter.

This commit is a mechanical change that uses the isolate available in the
context where String::Utf8Length() is called.

This is a breaking change imposing use of V8 6.9.411 or up.

[1] https://chromium-review.googlesource.com/c/v8/v8/+/1124724/
    https://chromium.googlesource.com/v8/v8/+/3dd5c6fe38355b8323597341409b37f931de5a85

* Remove the uses of deprecated snapshot-related functions

Due to V8::CreateSnapshotDataBlob() and V8::WarmUpSnapshotDataBlob() being
deprecated and altogether removed from V8 [1] it is necessary to migrate to
using local implementations of them.

This commit introduces create_snapshot_data_blob(), warm_up_snapshot_data_blob()
and the helper function run_extra_code(). Their implementations have been copied
over from [2].

[1] b3738e6583
    https://chromium-review.googlesource.com/c/v8/v8/+/1019442/

[2] https://github.com/v8/v8/blob/7.3.492.27/test/cctest/test-serialize.cc
    https://chromium.googlesource.com/v8/v8.git/+/30602560a8fdb0bbfb50d70be867f32b72758a2f/test/cctest/test-serialize.cc

* Non-trivial removal of uses of non-maybe Date::New()

Due to the non-maybe version of Date::New() being deprecated and altogether
removed from V8 [1] it is necessary to migrate to using the maybe version.

This commit introduces a context argument to the convert_ruby_to_v8() function
so it can be passed to the maybe version of Date::New().

This imposed changes throughout the code base so that the context can be passed
together with the isolate to convert_ruby_to_v8().

[1] https://chromium-review.googlesource.com/c/v8/v8/+/1357056
    e84b92d765

* Non-trivial removal of use of non-maybe Local<T>::ToString()

This commit builds upon fe62f7935582bd889742ec77ee2289a8f6cb16e6. The use of
Local<T>::ToString() in convert_result_to_ruby() did not have an immediately
available context to use. This commit unwraps the context from p_ctx and passes
it to the function and then unwraps the MaybeLocal result with
MaybeLocal::ToLocalChecked() assuming the verification beforehand is sufficient
to ensure there won't be any crashes.

* Update the changelog

* Replace placeholder in LICENSE.txt

LICENSE.txt

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2016 TODO: Write your name
+Copyright (c) 2016-2019, the mini_racer project authors.
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal