Module fxhash

Available on doc or test only.

fxhash modifications

(↑) This is derived work from the fxhash crate, including the following modifications:

• join all structs in a single HasherFx const-generic.
• implement the standalone functions as methods.
• make it no_std compatible.
• remove byteorder dependency.
• add const methods for hashing byte slices.
• rename the structures.
• misc. refactors.