Allow the creation of new Basic types in code

[vdka]

Allowing the creation of custom integer types at a user level will allow bitfields to be implemented nicely. This will be similar to how it is done in (ziglang)[http://andrewkelley.me/post/a-better-way-to-implement-bit-fields.html].

#import compiler("types")
u3 :: types.Integer(bits: 3, signed: false)
u2 :: types.Integer(bits: 2, signed: false)
BitField :: struct {
    a: u3
    b: u3
    c: u2
}