dillon1000/react

[rust][sema] Allow defining/resolving known globals

Adds an option to pass a list of known globals into the semantic analyzer so 
that references to globals can be checked. As a follow-up we'll need to 
distinguish between different types of semantic analysis errors, so that callers 
which don't want to validate globals can ignore "unknown variable" reference 
errors while still handling definite errors such as duplicate declarations.