When calling a method that accepts an anonymous splat and literal
keywords without any arguments, an assertion failure was previously
raised. Set rest_index to 0 when setting rest to the frozen hash,
so the args_argc calculation is accurate.

While here, add more tests for methods with anonymous splats with
and without keywords and keyword splats to confirm behavior is
correct.

Also add a basic bootstrap test that would hit the previous assertion
failure.

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>