Type checker

[JTrenerry]

Adds a new ProcCheck to ensure type correctness of an IR program.

• Change String.append to Printf.sprintf
• Descriptive error messages
• Write a test suite checking invalid types (Note: Some things are not tested i.e. calls since they fail at the parser and not the type checker)

[agle]

Name this as just TypeError or something, it would be nice to be more precise than a string but its fine for now.

[agle]

Conventionally you would also define a show_type_error type_error -> string function.

[JTrenerry]

Name this as just TypeError or something, it would be nice to be more precise than a string but its fine for now.

Okay, originally I did have stmt as well, but without position (within the stmt or the stmt in the ir) its strange.

(Could probably make this have stmt and stmt number (within a block) and block number, but unsure if there is a set order)

[agle]

Yeah at some point I'll need to set up the IR to pass through text locations from the parser for better error messages, for now i generally just use (block_id, stmt num) with stmt num being whats given by List.mapi ie indexes counting from zero

[JTrenerry]

Did this, but I moved a lot of functions under type_check which makes it pretty ugly, but was the easiest / nicest way I could think of passing the ids around without ruining the folds.

[JTrenerry]

todo: change return to use a pattern match instead of length for speed.