Cache Gem::StubSpecification#activated?.

this reduces about 10% total time of Kernel.require on my test environment.
Cache Gem::BasicSpecification#contains_requirable_file?.

this reduces about 10% total time of Kernel.require on my test environment.
Pass the full path when calling the original Kernel.require if available.

calling the original Kernel.require with a full path is about 25% faster than with path-less paramter.
this reduces about 5% total time of rubygem's Kernel.require on my test environment.
shouldn't be able to get the fullpath before activating.
Merge pull request #1060 from rubygems/speedup

Speed up rubygem's Kernel.require