Additional test cases on Integer#chr regarding invalid codepoints.

Additional tests have been written to ensure RangeError is raised on
a selection of invalid codepoints from UTF-8 and UTF-16.
Stricter test in String::from_codepoint regarding invalid codepoints.

Checking if ONIGENC_CODE_TO_MBC(...) <= 0 is not enough in discovering
if we're trying to convert an invalid codepoint into a string.
Using Encoding::precise_mbclen ensures the problem is caught and raised.
Merge pull request #2376 from razielgn/feature-improved-integer-chr-encoding-handling

Integer#chr raises RangeError on more invalid codepoints.
increment global serial if included module has constants
add spec for Module#include shadowing constants
Merge pull request #2378 from charliesome/invalidate-method-cache-if-included-module-has-constants

Invalidate method cache if included module has constants
Merge branch 'master' into concurrent_gc