Optimization: Value handling operations

[pepyakin]

Related to #99
Depends on #98

For now, functions like into_little_endian do an allocation. This is very unfortunate for the functions that are called upon every access to wasm memory.

At the very least, we can use change definition from

pub trait LittleEndianConvert where Self: Sized {
    fn into_little_endian(self) -> Vec<u8>;
    fn from_little_endian(buffer: &[u8]) -> Result<Self, Error>;
}

to something like this

pub trait LittleEndianConvert where Self: Sized {
    fn into_little_endian(&self, out: &mut [u8]); // Will panic if `out` of not appropriate size
    fn from_little_endian(buffer: &[u8]) -> Result<Self, Error>;
}

this will avoid allocations for into_little_endian case.