refactor: use libcbor instead of cbor-ruby

[grzuy]

This is a re-opening of #10 as "Draft".

Pending:

• Auto bytestring detection in libcbor-ruby (fixed in Automatically detect byte strings PJK/libcbor-ruby#10)
• Passing tests (waiting fixes for CBOR.decode("A\x00") returns empty string instead of "\x00" PJK/libcbor-ruby#11 and CBOR.decode("\xC6".b) raises CBOR::DecodingError` PJK/libcbor-ruby#12)
• Point to a released version of libcbor-ruby

[grzuy]

Pointing libcbor-ruby to master so that it has automatic bytestring detection (PJK/libcbor-ruby#10).

[grzuy]

After debugging further test failures here I filed the following in libcbor-ruby:

• CBOR.decode("A\x00") returns empty string instead of "\x00" PJK/libcbor-ruby#11
• CBOR.decode("\xC6".b) raises CBOR::DecodingError` PJK/libcbor-ruby#12

I think these are causing most of the pending test failures.

[grzuy]

This has faced too many obstacles and kind of blocked for a while now.

Closing for now.

Will consider re-opening when/if the following issues are fixed:

• Precompiled binary releases PJK/libcbor-ruby#9
• CBOR.decode("A\x00") returns empty string instead of "\x00" PJK/libcbor-ruby#11
• CBOR.decode("\xC6".b) raises CBOR::DecodingError` PJK/libcbor-ruby#12