implement float_to_int_unchecked
@hanna-kruppe would be great if you could have a look at this.
`float.rs` tests legal casts. `test_cast` checks that both `as` casts and unchecked casts work (i.e., these are not saturating). The `compile-fail` tests should ensure that illegal casts via the intrinsic are detected as such.
Fixes https://github.com/rust-lang/miri/issues/1264
