define `resize_capa` as a function

remove `mrb_str_offset`

remove `DBL_DIG` fallback definition

Fix MSVC warning for symbol.c

This increases readability and maintainability,
and it also doesn't use reserved identifiers.

Fix multiline comments

Speed up Array#select! from O(n^2) to O(n).

should care about the case that given arguments are less then mandatory formal arguments and optional arguments exist; close #2144

add post mandatory argument test

rename `mrb_str_buf_append` to `mrb_str_cat_str`

Fix MSVC warnings for numeric.c

Add Array#select_bang

Implement Struct#to_a and Struct#values .

The new name is better and less confusing, because:
  - `mrb_str_append` calls `mrb_str_to_str` and this function doesn't
  - `mrb_str_buf_append` _is_ `mrb_str_cat` for `mrb_value` strings

area of implicit nil is not enough to nregs