v0.0-806-g5c6ddec: SymbolTable and builder, first cut.

SymbolTable holds definitions and declarations.

Construct a symbol table for a single file.
(Multi-file merge to come later.)

This demonstrates how module is added to the symbol table.
Other language elements to follow later.

Next: separate Bindings structure for references
PiperOrigin-RevId: 347094159