Remove float, int, and uint as built-in types.

[shepmaster]

These went away pre Rust-1.0 and are valid identifiers. Went ahead and
removed them from the tests as well, replacing them with real types of
the same number of characters.

[shepmaster]

For reference, I was looking at some code like

pub use num::bigint::BigUint;

impl From<BigUint> for Literal {
    fn from(uint: BigUint) -> Self { Literal::Integer(uint) }
}

And was confused that the argument name and type were highlighted the same :-)

[tromey]

Thank you for the patch.