Change type parsing to be a bit cleaner

[alexdovzhanyn]

Currently types get parsed when they are written like so:

x<Function<Number, Number, Number>> = () -> ...

this works fine, but I think we should change the type declaration syntax to be a bit easier to read. It would be preferable to write type declarations like so:

x: Function<Number, Number, Number> = () -> ...

This is a little clearer as to what is the variable and what is the type.

• [] Update the parser to correctly parse type declarations in the new format
• [] Update all of the lexer, parser, typechecker, and codegen tests to use the new type declaration format. The logic of the tests themselves should not need to be updated.

[ohmschrodinger]

can you assign this work to me?

[alexdovzhanyn]

@ohmschrodinger you got it!