currently there are two scnenario to call mrb_run(), the first is
calling a proc, in this case mrb should create a new environment,
discarding all the variables except args, reciever and block.

the second is calling the newly generated irep, like in mirb.
in this case, the variables should be kept after mrb_run().
so we introduce mrb_context_run() to handle this seperately.

Added shuffle and shuffle! to the Array class in the Random gem.

fix 3a4c8e2d

unfortunately the previouse commit is incorrect, which still concate the
keywords, but like this:

      method='"end\nb"' (201)

fix #1550

fixed compilation on freebsd releases witout log2 function

emit a "\n" as the first token for parser instead of taking the
first character from the next file for lexer, to prevent mruby
"concatenate" two keywords between files

it should be regarded as a work around, for it can not resolve
this case:

$ cat a.rb
"
$ cat b.rb
b"

$ bin/mrbc -o- -v a.rb b.rb
mruby - Embeddable Ruby  Copyright (c) 2010-2013 mruby
developers
NODE_SCOPE:
        NODE_BEGIN:
                NODE_STR "

b " len 4
irep 0 nregs=2 nlocals=1 pools=1 syms=0
000 OP_STRING   R1      "\n\nb "
001 OP_STOP

thanks @bovi 's idea

Disable implicit integer conversion in mrb_get_args

We have already reserved that register in codegen()'s case NODE_DEF, but
what we care about is actually the previous register. So what we need is
get that register by cursp() after pop(), then recover the reservation
by push().

fix #1542

Set regs to stack on return of const_get calls before use

Add some methods to Symbol

remove unused value for mrb_gv_remove