Reopens #3612.

Reopens #3550. Those 2 issues are exclusive.

Support `freeze` method in `Struct`.

Fix typo in usage message.

Align pool allocator with 8 byte in 64-bit environment.

Add missing `arg_i++` to fix bug of `s!` in mrb_get_args.

To avoid warnings from undefined sanitizer.
( https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html )

A new function `mrb_yield_cont()` is provided. You have to call it
at the end of a C defined method, e.g. `return mrb_yield_cont()`.

Some examples in #3359 still behave differently from CRuby.

Omitted `goto L_RAISE` to raise an exception.

Check if sc->mt is initialized before copying it.

Shared TT_ENV should be unshared. Reversed condition.

The fix was wrong. It causes a new problem #3612.

So that `yield expr_with_error` will cause the error from the
argument rather than `LocalJumpError` when no block is given.

Otherwise line number information is lacked from exceptions
raised in VM, e.g. "super called outside of method".

ref #1359 #1362

Fix an off-by-one causing a buffer overflow in mirb.

One for the receiver, the other for the block.