Added CashAddr benchmarks for decoding/encoding

Summary:
Added CashAddr benchmarks similar to the bech32 benchmarks in Core's PR 13586
Completes T419

Test Plan: make check && bench_bitcoin

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1856