First class functions, function types, conversion functions

[sharkdp]

This PR implements function types and function references (first class functions). It also allows single-parameter functions to be called using x -> f as an alias for f(x). Both things together enable a lot of interesting features:

• We can implement functions like fn diff(f: Fn[(Scalar) -> Scalar], x: Scalar) -> Scalar in Numbat
• We can write 10 million seconds -> human, 78 -> chr or now() -> unixtime instead of using the // operator or normal function call syntax.
• We can return functions from functions. For example, we can return a base-b conversion function from base(b), allowing us to write things like 0xffee -> base(2)
• We can simply type the name of a function to see it's type signature

Limitations:

• This does not (yet) work with generics

To do:

• Disallow taking function pointers to generic functions (for now)
• Disallow function types with unknown types (no support for generics for now)
• Proper type check for function pointer calls
• Allow FFI functions to be the target of a function pointer
• Re-enable suggestions for wrong function names ("Unknown callable: Cos, did you mean cos")
• Resolve all todo!s
• Resolve TODOs
• Add tests
• Allow returns_fn()(…) input? Currently only works using (returns_fn())(…)

closes #202