std.hash.crc: get rid of usingnamespace

This flips things around such that std/hash/crc.zig is generated
by the catalog-based generation tool, and the real code that used
to be in that file is moved out to std/hash/crc/impl.zig.  The
generated tests are moved to std/hash/crc/test.zig.  By going this
route, we eliminate the need for usingnamespace without changing
anything for callers of these interfaces.  The Crc32 tests are
simply added to the fixed part of the generated output and
compactified a bit.

This was the second-to-last usage of usingnamespace left in std.

Author: blblack