Remove separate file loaders and replace with one

All tests updated to work with the new simplified loader
Simple function declarations (no callbacks)
Merge pull request #245 from cyli/ssl_simple_function_bindings

The simple function bindings for openssl ssl.h
Merge pull request #244 from reaperhulk/less-loaders

Remove separate vector file loaders and replace with one
Simplify and make better hte tests for load_from_file
simplify
fixed bytes
flake8
Fixed using copied hashes
Merge pull request #247 from alex/fix-hash-copy

Fixed using copied hashes
Merge pull request #246 from alex/delete-tests

Simplify and make better hte tests for load_from_file
Get a HashContext from the backend like we do a CipherContext
Reflow Hashes.supported.
GCM loader support
Merge pull request #182 from reaperhulk/gcm-loaders

GCM test vector loaders + tests
Merge pull request #248 from dreid/hash-context-from-backend

Get a HashContext from the backend like we do a CipherContext