Option for restrict map keys

[methane]

While msgpack spec doesn't prohibit any map keys, some languages allows only str as map keys.
So it's recommended for cross language compatibility.

For packer, it is useful to detect (unexpected) bin keys.
For unpacker, it is important to prevent "hashdos" attack too.

• 0.6: Add strict_map_key option to packer and unpacker. False by default.
• 1.0: Make it true by default.

[methane]

I decided to not adding the option to packer.
It's security feature of unpacker.