Focus was on the front end (not SPIR-V), minus the grammar.
Reduces #ifdef count by around 320 and makes the web build 270K smaller,
which is about 90% the target size.
The grammar and scanner will be another step, as will the SPIR-V backend.
This makes heavy use of methods #ifdef'd to return false as a global way
of turning off code, relying on C++ DCE to do the rest.
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