The POSIX `realloc` keep the original pointer untouched, so it can
easily leads to memory leakage. `mrb_realloc()` should handle those
bookkeeping, while `mrb_realloc_simple()` keeps the original `realloc`
behavior.

Use simpler `mrb_get_argc()` and `mrb_get_arg1()` instead.

C implementation used `mrb_funcall()` that bypassed many optimization.

`mrb_get_arg1()` raises `ArgumentError` if the method does not receive one
argument.

And replaces all `mrb_get_args(mrb, "o", &arg)` by the new function.

Since we made our opcode byte based, we need no endian option (`-e`
and `-E`) to `mrbc`.

Remove `mrb_assert()` in `Module.nesting`

Need `build/XXX/mrbgems/mruby-test` directory

Update `test/t/kernel.rb`

The following code was causing SIGSEGV:

```ruby
Module.method(:nesting).call
```

An error may occur when performing `rake clean all` or `rake clean test`.

The directory is needed before writing to `mrbgems/mruby-test/assert.c`.

- Remove unnecessoury assert
- Fix typo

only use GEMNAME if defined in assert.rb

Check if the constant GEMNAME is defined before use in `assert.rb`.

This is added to prevent an undefined constant error when using
`assert.rb` in other environments - for example, testing CRuby.

Print exception backtrace if possible without `-v` in test

The issue was reported by @shuujii.