fix signature of mrb_class_new_instance()

fix String#inspect with MRB_UTF8_STRING

dd925578 changed the order of arguments,
but it doesn't seem intentional.

cf. #2963

define MRB_UTF8_STRING (in mrbconf.h) to enable UTF-8 support.

C API documentation

some routines need to refer mruby objects (e.g. callbacks), in that case
you have to protect your objects from garbage collection. the new functions
mrb_gc_register() keeps those objects from GC. you have to remove your
objects using mrb_gc_unregister() when your C routines use mruby objects
any longer, otherwise objects will leak.

Fix indents

Replace 1 with TURE macro

Problem: Hash#fetch doesn't raise KeyError when a key cannot be found

Solution: change the Exception class raised to KeyError when a key
cannot be found.

support String#[pos, len]= val

fix block variable in Hash#fetch

Don't crash if NULL is passed to mrb_close

target_class should be singleton class of the receiver

Sometimes it is very useful just return from mrb_close if NULL is
passed as mrb.  This is the same spirit of free(3), which just does
nothing if NULL is passed.