Defer integer literal type resolution until after type checking

[mohammadfawaz]

Fixes #653
This patch basically tries to remove the assumption that integer literals are 64 bit wide. This implies the following:

1. Writing let x:u32 = 5; does not generate a warning.
2. Writing let x:u8 = 256; generates an error because 256 does not fit in a u8.
   I was hoping to completely remove implicit casting between UnsignedIntegers in engine.rs, but there are more complex situations for which resolving the type of literals without the 64-bit assumption is not yet possible. E.g.:

struct Foo<T> {
  a: T,
}

fn main() {
  let foo = Foo { a: 256 };
  let x:u8 = foo.a;
}

Ideally, we would recognize 256 has to be a u8 and error out, but that requires more complex analysis than what I have in this change.

[adlerjohn]

Ideally, we would recognize 256 has to be a u8

You mean u16?

[mohammadfawaz]

Ideally, we would recognize 256 has to be a u8

You mean u16?

Oups - yes with 256 we should really get an error. 255 is okay. I updated the description, thanks.
E.g.: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=dd4566ec8364d4fdeb15615d75e5f581

[sezna]

approved w/ nit -- up to you whether or not to address it.