flua: Add hash module

[imp]

Summary

Add the hash module, and the sha256 method. We support creation,
update(), digest(), and hexdigest() methods that are all thin wrappers
on SHA256_Init(), SHA256_Update(), SHA256_Final() and SHA256_End()
respectively. update() returns the underlying object so it can be chained:
hash.sha256():update("F"):update("r"):update("e"):update("d"):hexdigest()
is the same as hash.sha256("Fred"):hexdigest().

Sponsored by: Netflix

[imp]

Don't use SHA256_End. We don't have it in the boot laoder
and it's way easier to expand that inline than hook that file
into the build.

[kevans]

libexec/flua/Makefile

21–22

Do you envision needing this in bootstrap flua, or would it be OK to move this out to it's own module (lib/flua/libhash)? The only requirement on a module is that it exports its luaopen_*() that matches the module's name and be installed in the right spot, then it should Just Work(TM).

libexec/flua/modules/lhash.c

41

It would perhaps be a bit more efficient to instead lua_settop(L, 1) to trim the stack back to the context

132

This and the statement after can be combined into a single luaL_setmetatable(L, SHA256_META) (it will set it for the object at the top of the stack)

[imp]

libexec/flua/Makefile

21–22

I'll have to give that a try. I'll have to change how I'm building the loader patch, but that's ok...