Improve handling of "exit" and "quit" in mirb. 

function prototypes sorted by call & removed unused (file, io, thread)

khash.h including at line:255

Modify buffer size for filename.

Code formatting in sprintf.c

Also, fixed issue in Xcode project build settings

switch/case constructs in mrb_str_format() had mismatched indentation
styles: clean up to 2-spaces-per-level.

Canonicalize NaN value in NaN boxing environment

op_enter_optimize

Xcode project cleanup (corrects the error in #422, not pulled)

Now there are only three schemes (build commands, static library, OS X
framework) plus a scheme wrapping the standard make file.