Also enables debug symbols in release profile by default. Until
Alacritty ships, there's going to be lots of perf analysis which needs
debug symbols.
The PriorityMutex low priority method was never used. Now it's just a
fair mutex.
The pty read thread now runs the parser and directly updates the
terminal in the same thread. This obviates the need for a channel which
sends every char read from the pty; this is a huge performance boon.
Synchronization between the updater and the renderer is now achieved
with a PriorityMutex. Previously, an atomic bool was (poorly) used to
request the lock on terminal. The PriorityMutex is dead simple to use,
and it _Just Works_.
Joe Wilm