singleton class needs to set __outer__; fixed #1983
use size_t instead of int

Conflicts:

	include/mruby/khash.h
too much peephole optimization; close #1992

Conflicts:

	src/codegen.c
Support :T_FIBER in ObjectSpace.count_objects .

Conflicts:

	mrbgems/mruby-objectspace/src/mruby_objectspace.c
add tests for FalseClass

reorder and add tests for 15.2.6.1