Type checking of expressions / functions!

This is a bit of a chonky commit, but it adds in the basics of checking
the types of expressions / function calls / return types. There's still
a lot of work to be done, including:

(1) Adding new core types, and casting between allowed types
    automatically
(2) Picking the corrent output type based on input types (for instance
    float+int == float)
(3) We need much better error reporting, the error messages are really
    vague and unhelpful as-is
(4) We also need to work to ensure that a function with a return type
    actually returns
(5) Possible re-factoring to make stuff less hacky when we have more
    types / structs / arrays / etc.

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