Remove unused rb_str Rubinius specific functions

rb_str_update() also collides with an actual Ruby C-API function, also
see #2459.
Add basic spec for rb_str_update
Add better inspect for Rubinius::Mirror
Add rb_str_update

Fixes #2459
Fix style of always using {}
Setup 1.9 aliases for rb_str_new4

In CRuby rb_str_new4 and rb_str_dup_frozen are aliases for
rb_str_new_frozen, but that would mean a bigger change since 1.8 mode
still uses the old non descriptive name rb_str_new4. So for now, we just
leave it at this.
Add C-API header updates to 2.0 mode
Force reconfigure

I wasn't ablt to build Rubinius anymore, took me a while to realize I
should try a reconfigure. So make we force this so people who already
have a checkout rerun configure.