Proto: parse addresses "addr%" as "addr"

Context

Fixes: #643 (closed)

#643 (closed) revealed that addresses with empty entrypoints (like "addr%") are allowed in Readable notation. Upon PACK, they collapse to addresses with no ("default") entrypoint (like "addr".)

This change attempts to fix the problem by parsing "addr%" as "addr".

Manually testing the MR

(cd src/proto_alpha/lib_protocol/test && dune exec ./main.exe -- test "typechecking") # ??

Reviewers

@rafoo_ @klakplok @mbouaziz ?