xwillfree - promise to free memory

This patch changes semantic of RUBY_GC_MALLOC_LIMIT.
Instead of being "periodical trigger" it becomes more like "safety trigger"
which fires in allocation increase (instead of allocation amount).
So that there is less need to tune RUBY_GC_MALLOC_LIMIT at all, and default
8Mb becomes meaningful.

`make check` runs 13% faster (292s instead of 338s) and doesn't seems to
use more memory.

Tradeoffs for patch simplicity:
- it is not exact: only String, Array, Object, Struct, Bignum and Time are
  handled
- only one function (xwillfree) introduced. Perhaps, more readable api
  could be useful.
- xwillfree exposed to the public (ruby.h). Perhaps, it should be in an
  internal.h, but st.c doesn't include internal.h .
  But may be it could be useful for extensions.