Remove use of unsafe transmute in read_f32 and read_f64

[sunfishcode]

wasmparser currently uses an unsafe transmute to convert a floating-point literal from raw bytes to an f32 or f64. In practice this is fairly safe, however it's desirable to avoid unsafe code in general.

It's tempting to convert these to the new to_bits/from_bits APIs in Rust once they become stable, however those APIs have the subtle behavior of quieting signaling NaNs, making them unusable for wasm decoding.

Cretonne solves a similar problem by representing floating-point literals as integers holding the bit patterns. I think that would work well for wasmparser too, since it isn't doing any of the actual floating-point arithmetic itself, so it doesn't need the values in f32 or f64 format.

[yurydelendik]

Fixed by 7e8c792