For readability's sake.

There's a number that negation does not work (-2147483648 in 32bit
environment).

Since there's no unsigned integer in Ruby. Binary, octal and
hexadecimal negative numbers can be represented by using 2's
compliment. But decimal (not being power of 2) cannot be use
that kind of format.

mruby-1.3.0

Now you can just call `mrb_load_*` functions then check
`context->parser_nerr > 0` if the return value is `undef`,
instead of calling `mrb_parse_*` functions independently.
ref #3248 #3331

This reverts commit 7944d9a6.
Because it voids #3248 and #3331. But we should add better way
to check whether compile errors occur without duplicated callings.

As a result, `#chr` is not called for ch < 0x80, so we need to
update the "invalid chr" test.

It's incompatible with CRuby, but it is required for mruby because
it cannot detect `break` outside of loops in the parsing time, but
in the code generation time where it cannot raise `SyntaxError`.

#3711 fix

Save gc_arena after buffer got allocated

Latest fix was freeing the buffer while it was still in use.
Fixes #3712