The F# Build should cache the generated lexer/parser instead of recreating them all of the time

**Is your feature request related to a problem? Please describe.**

If the generated lexer and parser were checked in then they wouldn't need to be re-built quite so often. The files are almost perfectly cache-able (the only inputs are the lex/yacc binaries and the fsl/fsy file inputs) and this generation would mean that we don't have to

* rebuild the packaged lexx/yacc
* regenerate the parser/lexer

Running this is a cost that every proto build has to pay, for no good reason. Generating the lexer and parser on my machine takes ~12s:

However, it also takes ~35s to build fslex and fsyacc projects - costs that could be avoided if the files don't need to be generated.