Add Module::new_unchecked to avoid expensive Wasm function validation

[Robbepop]

Users of wasmi might sometimes know ahead of time that the binary blob that they are working on has already been validated to be a proper Wasm binary. In these cases wasmi currently does not provide an API to avoid the duplicated Wasm validation since Module::new always validates its input bytes.

This issue proposes to add

impl Module {
    pub unsafe fn new_unchecked(engine: &Engine, wasm: impl Read) -> Result<Self, Error> { ... }
}

Note that we still return a Result<Self, Error> since this does not drop all checks, only the very expensive checks from validating Wasm functions.

Together with lazy compilation this should provide a good amount of speedup when working under these conditions for users of the wasmi crate.