replace quicksort with mergesort.

Refactor .travis.yml to remove duplicated env: MRUBY_CONFIG.

That means entry table should be compacted periodically by `sg_compact()`.

I know it's not hash at all, but reduce memory consumption.

This reverts commit 7b04fcd0.
The issue was addressed by 9e3cbaaa. No longer needed.

Fix memory leak in `mrb_str_count`

Fix comparisons in str_squeeze.

* Add option to disable direct threading

* Prepend MRB_ to option name

`OP_STOP` returned a wrong value.

mruby restriction:
In mruby, `String#delete` only takes single pattern argument.

mruby restriction:
In mruby, `String#count` does not take multiple pattern arguments,
but only one pattern.

mruby restriction:
`String#squeeze` can take more than 1 pattern arguments in CRuby,
in that case, the intersection of patterns will be used to match.
But in mruby, it doesn't take multiple patterns.

This patch is based on `mruby/c` implementation by Hirohito Higashi.

We might need to add `#tr_s`, `#squeeze` and `#delete` as well.
Adding them should not be too hard using functions we implemented here.

This reverts commit 1997fcf9.
It became the default behavior.

Merge branch 'assign-anon-class-name' of https://github.com/dearblue/mruby into dearblue-assign-anon-class-name

Even though `Symbol#to_proc` is not included in ISO standard, the
`some_method(&:method_name)` is used very widely and convenient.
So we moved it to the core.