Merge /api/improve and /api/alternatives, remove custom Herbie properties

[pavpanchekha]

The core idea behind this PR is that both of these APIs return JSON blobs, and those JSON blobs have distinct sets of keys. So we can just merge both endpoints and return both sets of keys, and everyone will be happy.

Doing this is quite involved, and required:

• Deleting alternatives in favor of improve
• Adding alternatives, histories, and derivations keys to make-improve-result
• Dropping :herbie keys from unparse-result
• Dropping unparse-result in favor of alternatives (which is an FPCore)
• Fixing the alternatives key in case of a string name with spaces