Get rid of assert_nothing_raised

Uses were replaced with a simple 'pass', or a better assertion. Some
tests were updated to match the current implementation. Some tests were
removed.
Fix some indenting
Move definition of GObject.type_init to gir_ffi-base

Due to this change, GObject.type_init can be used directly in
GObjectIntrospection::IRepository.
Get rid of assert_not_nil
Get rid of teardown and context
Get rid of `should` as an alias for `it`
Get rid of `setup` as an alias for `before`
Remove a TODO
Hide ITypeInfo#flattened_array_type that's only used internally
Remove extra local variables
Update dependency on Rake
Remove ReturnValueBuilder#cvar