Remove dependency on Half only where obsolete

[dabrahams]

Description

This is just #21 but with Half still in use on x64 Macs. I need to remove the Half dependency in order to run on Windows but unfortunately x64 Macs (only!) don't support Float16.

Fixes #29 by avoiding SomeRandomiOSDev/Half#16

Checklist

Ensure that your pull request has followed all the steps below:

• Code compilation.
• All tests passing.
• No new SwiftLint issues.
• Added new unit tests, if applicable.
• Extended the documentation (including README), if applicable.
• Updated version in CBORCoding.podspec following semver guidelines.
• Ran workflowtests.sh and passed.
• Added myself to the CONTRIBUTORS file.

workflowtests.sh fails, but it already fails for me on yourmain branch. I'll report a separate issue.
I'm already in CONTRIBUTORS.md

Proposed changes

In addition to the motivation above, see #21.

[SomeRandomiOSDev]

@dabrahams I created a release to address a lot of the issues that came up due to this repo being so out of date. Please rebase on this release and update this PR accordingly

[dabrahams]

@SomeRandomiOSDev Rebasing is going to be tough. I'm just trying to get as far as making a merge work; I'm not an Xcode/carthage/cocoapods/whatever jock, so making all those parts work is a challenge for me. It's not clear whether you're willing to accept the minimum deployment targets advancing considerably in order to get Float16 supported, but that's what @christophhagen seems to have assumed and I'm following the same pattern.