Rename radix typename for key type to something searchable

Summary:
This makes it easier to skim over radix.h for uses of KeyType since "K" gives more false positives than hits.
Doing so helped me better understand the interfaces at play in RadixTree. I expect this patch will assist others in the
same way.

The naming convention was selected based on existing usage in the codebase. See examples with:

grep -hrio "typename [a-zA-Z]*" | sort | uniq

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11407