* Avoids boot time mutation of REGISTER constant
* Allows to define project specific expression parsing
* Avoids custom (slow) serialization of Expression objects speeding up
reporter / killer IPC
* Improve specification
* Improve integration API as it now finally references an object the config
* Allow reproduction of syntax from Expression#syntax
* Allow instantiation of Expresssion objects without generating the
syntax, much nicer for most specs & internal code, avoids generating
a string to parse it into an expression
* Fix LSP violation in Mutant::Matcher namespace
* Before Mutant::Expression was not marshallable resulting in the need
to build "partial" result objects from killforks.
* Fixes an adjacent bug in spec selection
