Font: Add clone constructor
MRI-Binding: Bind #initialize_copy for clonable classes

This replaces the previously directly bound #clone
methods, which weren't really the "the Ruby way".
Rubys default Object#clone will call into our #init_copy
methods instead.

Partly incorporates pull request #3 by /cremno.
MRI-Binding: Catch exceptions in clone constructors
Bitmap: Ensure cloned bitmap is non-mega
Filesystem: Optimize RGSSAD file enumeration

Finally got around to nuking that ugly pile of shit that was
previously there for PhysFS file enumeration because filepath
cache generation with unencrypted game files + archive + RTP
has started taking around 6 seconds. Thank $DEITY.
Readme: Fix typo
MRI-Binding: Bitmap: Add FIXME