array#select!, keep_if
Implement Numeric#abs2

`self ** 2` is failed on RubySpec.
Because calling Numeric#* is a spec.
Merge pull request #629 from kachick/implement-numeric-abs2

Implement Numeric#abs2
Fix Array#[]= for frozen objects
Merge pull request #630 from kachick/fix-array-subscript_assign

Fix Array#[]= for frozen objects
Implement String#casecmp
Tag a bunch of new tests
started implementing Kernel#__method__
Module.nesting and Module#remove_class_variable
added flags on modules
strings can be symbols (sometimes?)
Make flags and duping work on Modules
error condition on Module#module_eval
Stub out Module#private_constant
tag a bazillion specs that now run
Skip another test.
Merge pull request #623 from kachick/implement-string-casecmp

Implement String#casecmp
Merge pull request #569 from kostya/select_i

array#select!, keep_if
Implement String#start_with?
Implement String#end_with?