define `resize_capa` as a function

This increases readability and maintainability,
and it also doesn't use reserved identifiers.
include the missing header for `DBL_DIG`
remove `DBL_DIG` fallback definition
fix disabled stdio (and remove trailing spaces)
remove `mrb_str_offset`
Merge pull request #2154 from cremno/dbl_dig

remove `DBL_DIG` fallback definition
Merge pull request #2157 from cremno/remove-mrb_str_offset

remove `mrb_str_offset`
Merge pull request #2155 from cremno/define-resize_capa-as-a-function

define `resize_capa` as a function
Merge pull request #2156 from cremno/fix-disabled-stdio-2

fix disabled stdio
turn on MRB_GC_STRESS for debug build on travis