Reorder members in `struct backtrace_location`

Remove unneeded `memset()` in `src/backtrace.c`

`sizeof(struct backtrace_location)` is 24 bytes -> 16 bytes in LP64
data model etc.

Fix destroyed "inline packed symbols" on 32 bit mode with `MRB_WORD_BOXING`

Refine dependencies for `mruby-config`

Fix typos

Add length argument for `sym_inline_unpack()`

`sym_inline_unpack_with_bit()` is moved inside of `sym_inline_unpack()`
because this is used only one place.

Extract code fragment for unpacking into method in `sym_inline_unpack()`

Use `mrb_assert()` for checking `sym` in `sym_inline_unpack()`

Add `const` to `pack_table` in `src/symbol.c`

Lazy message/diff creation for assertion in `test/assert.rb`

Include the following changes too:

- Extract part of logic with block to a method
- Use `var ||= ...` instead of `var = ... unless var`
- Unify using parentheses for `t_print`
- Change methods order

Always through `assert_true` for assertion methods in `test/assert.rb`

Refine `mrbgems/mruby-bin-config/mrbgem.rake`

- Use `MRuby::Gem::Specification`
- Fix a binary name is added to `MRuby::Build#bins` multiple times
- Close file immediately (avoid using `open(...).read`)
- Simplify

Small symbols with all alphanumeric characters (<5) are packed in 32bit
symbol integer a la base64. This means those small symbols are not
listed in `Symbol.all_symbols`.

Small refactoring in `codegen.c`

NULL safed `mrb_open_core()`

Add `NoMethodError` tests to `mruby-enumerator`