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.

Fix outer module name

The ISO standard does not include implicit type conversion using
`to_int`, `to_str` and sometimes `to_f`. For the compactness of the
mruby implementation, maybe we should remove those implicit conversion
from mruby.

fix #4116; fix #4117; fix #4118; fix #4119; fix #4120

Exclude pipe and exec on iOS/tvOS

Use of `execl()` is prohibited on these platforms

Prevent signed integer overflow.

Fix typo

example:
  a = Module.new
  a::B = Module.new
  p [a, a::B] # => [#<Module:0x801825890>, #<Class:0x801825890>::B]
              # => [#<Module:0x801825890>, #<Module:0x801825890>::B] (Fixed)

example:
  a = Class.new
  a::A = a