Use mrb_ptr instead of mrb_cptr in Kernel#to_s

Fix some incompatibility for Enumerator#with_index

Fix segfault in mrb_proc_copy.

Fix more integer overflows.

This is to avoid segfault when WORD_BOXING is enabled

Reported by https://hackerone.com/brakhane

Feature/multi unicode escape

Add test for recently fixed bugs

Prevent array size calculation overflows.

Support Enumerable methods

Implement Enumerable::Lazy#to_enum and enum_for

Lazy class should be under Enumerator instead of Enumerable

Copy over INSTANCE_TT when duping class

Use size_t to calculate bytes needed for array.

Interpret argument in 'f ()' as nil instead of nothing