Descriptive variable names.
Don't allow break outside loops during typechecking.
Rename endWhileBlock endLoopBlock.
Compile with clang.

Enable -Weverything with some exceptions.
Remove unused 'last token' functionality.
Move library includes into common_system.hpp and use pragma
system_header to disable warnings.
EInteger now holds an unsigned. The parsed value is never negative.
Move AddBuiltins prototype to common.hpp.
A few other warning fixes.
Implement return. Unify type of return expressions with type of body.