Determine type of strings with bidirectional type inference

[Akuli]

Sounds fancy, but it isn't. It just means that when we are type-checking an expression, we have some idea about what we expect the resulting type to be.

For example, if you do foo: byte[100] = "hello", when we type-check the string "hello", the type of the string becomes byte[100]. But if you do foo = "hello" without specifying a type, the type of the string becomes byte*.

Fixes #683