Changed the RData type field to 'const'

Within the mruby source code the 'type' field is either initialized, or used in const manner.
IMHO changing the type after it has been created might cause hard to track errors.
Include stddef.h. It is required by size_t. In case you include stdio.h, stddef.h is included indirectly.
Separate FILE dependencies with ENABLE_STDIO.
Reduce using snprintf(). They can replace by mruby API.
Add stdio.h. This is just for now. We should discuss about remote tests later.
Merge pull request #1061 from nemerle/RData-type-constification

Changed the RData type field to 'const'
Make topenv() static inlined
Merge pull request #1066 from xxuejie/static_inline_top_env

Make topenv() static inlined to avoid compiling error when using clang and -O0 mode
Fix indentation in mirb.c
mruby-sprintf should precede mruby-print
Merge pull request #1070 from mattn/fix_order

mruby-sprintf should precede mruby-print
Merge pull request #1069 from kano4/pr-fix-indentation-in-mirb_c

Fix indentation in mirb.c
Merge pull request #1065 from monaka/pr-reduce-stdio-dependency

Reduce stdio dependency
remove unused functions 'mrb_strerrno' and 'mrb_bug_errno'.
Merge pull request #1071 from crimsonwoods/remove_strerror

remove unused functions 'mrb_strerrno' and 'mrb_bug_errno'.