Implement semantic analysis

[Dandigit]

The old analyser module needs to go, particularly given the significant language changes that have occurred. I did start working on a sema module; I intend to pick up where I left off with that.

One pass over the AST (as was conducted by analyser) will not suffice. The minimum number of passes I think are reasonable is somewhere around 3, which is unfortunate but necessary.

• Collect impl blocks and associate them with structs
• Declare all variables and types in the symbol table
• Attempt to associate all symbols with a value

This should allow for declarations in any order. I'm considering experimenting with bidirectional type checking, so that generics are a bit more elegant. Consider:

func cast(x $From) $To => x as $To;
func takesAnInt(x int) int => x;

// Without bidirectional type checking
takesAnInt(cast(2.5) as int);

// With bidirectional type checking
takesAnInt(cast(2.5));