take better advantage of precompilation

[Steven G. Johnson]

Currently, there are a lot of things happening in the __init__ statement, which makes precompilation not help much for Nettle. I think this can probably be fixed.

e.g. consider hash_init. Instead of defining a new type <: HashAlgorithm for each hash Nettle defines (detected at runtime), why not just work with NettleHash directly, with a constructor like NettleHash(name::AbstractString) to look up the structure at runtime for a given name, and a function nettlehashes() to return the available names?

In most applications, like IJulia, we only want a particular hash, and we know which one we want in advance — if Nettle is missing this one, we just want to throw an error. So, there is no point in setting up all the hashes when the module is loaded.

[Elliot Saba]

Closed by ed4d932, although a better selection of functions in precompile.jl wouldn't go amiss.