MEDIUM: proxy: index proxy ID using compact trees

MEDIUM: proxy: index proxy ID using compact trees

The proxy ID is currently stored as a 32-bit int using an eb32 tree.
It's used essentially to find holes in order to automatically assign IDs,
and to detect duplicates. Let's change this to use compact trees instead
in order to save 24 bytes in struct proxy for this node, plus 8 bytes in
the root (which is static so not much relevant here). Now the proxy is
3088 bytes large.