`mrb_str_buf_new` is an old function that ensures capacity size of
`MRB_STR_BUF_MIN_SIZE` minimum. Usually one need to use
`mrb_str_new_capa` instead.

According to the naming convention a function that implements
a method should be suffixed by `_m`.

Reset ci in OP_SUPER after potential realloc

Check whether internal khash is initialized in Hash#compact!

Improve Array structure

Some more vm refactorings

Because of #3504 `ci->nregs = bidx+1` was introduced in b64f0878.
This led to the follow up error #3551 whose fix introduced the `if (bidx >= ci->nregs)`
check in 071164b7 and the `stack_extend(mrb, ci->nregs)`
in 93d80298.

Then, the code causing #3504 reappeared again in #3590. The fix for it moved the code
dealing with the block in OP_SUPER from below the `cipush`  to above the `cipush`
in d9fb8b69. The `if (bidx >= ci->nregs) { ... }` from
then on works with the original callinfo and not the pushed one. `ci->nregs` needed to
be modified for the pushed one because it is initialized to 0. But for the original ci
it is propertly set and a check is not needed.

Refactored OP_SEND and OP_SUPER and calculate argc right at the top

Added basic test for calling a missing method through super

The error checks for both usage should be separated; ref #3774

Merge branch 'super_method_missing_fix' of https://github.com/christopheraue/mruby into christopheraue-super_method_missing_fix

And the function does not conform the naming convention anyway.