Track symbol references

[DonIsaac]

Record where symbols are referenced and how they are used.

Create a Reference struct and store it in the SymbolTable. It should include the following fields

• node_id: the Ast.Node.Index of the AST node making the reference. Note that .identifier does not have its own AST node.
• token_id: the TokenIndex of the .identifier token.
• symbol_id: The symbol being referenced. null if it could not be resolved.
• flags: ReferenceFlags describing how it's being used. This is a packed struct (i.e. a struct only containing bitflags) with these fields:
  • read: symbol's value is being read. Also set for LHS of field_access nodes.
  • write: symbol's value is being modified.
  • call: symbol is being called.
  • type: symbol is used in a type alias (const Foo = std.ArrayList(Bar)), function signature, or type annotation.

Acceptance Criteria

• References are stored as an SoA within SymbolTable
• yet-unresolved references are recorded by SemanticBuilder. Each time it exits a scope, it should attempt to resolve references.

Reference flags edge cases:

• given var x = 1; x += 1, x is both .read and .write
• given foo.bar(), foo is only .read, while bar is .call

TODO

• add .type flag to type references
• visit variable type annotations
• handle .alias reference flags
• add builtin flag? add it to null, undefined, builtin types, and builtin functions?
• handle .member reference flags. I don't think field accesses are bound correctly yet.

[DonIsaac]

#90
#94