feat: implement semantic analysis

[DaniPopes]

• feat: allocate the AST using an arena #32: Closes Investigate flattening the AST #15
• perf: use smallvec in the parser #36: Closes Use SmallVec more often in the parser to avoid small heap allocations #33
• feat: finish symbol resolution #42: Closes Add AstPath #41
• feat: reduce sizeof::<TokenKind> from 12 to 8 #46: Closes Reduce the size of TokenKind to 8 bytes #28

[DaniPopes]

This is not finished, will continue in another PR for a smaller diff