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

Define eval as module function.

correct to compare MRB_STACK_GROWTH.

Refactor numeric.c

Refactoring Comparable

Add Array#keep_if