fix: correct stack value handling for nil, multi-returns, and NUL bytes

[veelenga]

Overview

Fixes three pre-existing bugs found during the review of #45:

• Stack#pop now removes nil values instead of leaving them on the stack, which corrupted results and leaked stack slots (run(%q( return 5, nil )) returned [nil, nil] and left a dirty stack)
• pick_results reads values by index instead of popping, so multiple return values keep their order (return 1, 2 returned [2.0, 1.0]); coroutine resume now locates results via the nres out-param of lua_resume
• strings with embedded zero bytes survive both directions: reads use tolstring's length out-param and pushes use pushlstring with bytesize (previously truncated at the first \0)

Each fix comes with regression specs.