Fixed masgn post-argument ordering.
Added post variables to masgn to_sexp.
Fixed block variable shadowing.
Fixed to_sexp for Hash splats.
Fixed bytecode for **var in Hash literal.
Fixed bytecode for SplatValue.

This bytecode may be replaced with a future instruction or set of instructions,
but cast_array semantics are incorrect here and cast_array is used in various
locations, so until all those are audited, this is simplest.
Fixed bytecode for masgn.

This code is correct according to the rewritten RubySpecs. Subsequently,
we'll address the duplication and possible instruction fixes or additions.