Optimize SigV4 and add S3 content-sha256 handling

Switch signing to in-place strided buffers in SigningResources, removing
the per-call TreeMap/ArrayList/Pair allocations and one UTF-8 encoder
pass. Add x-amz-content-sha256 to the canonical request and wire request
for s3/s3express; honor pre-set X-Amz-Date, security-token, and content-
sha256 headers; throw on unknown-length payloads instead of signing as
empty; treat port -1 as default in the Host header; fast-path the
already-canonical query string. Test runner ignores Content-Length when
comparing requests (signature-irrelevant; auto-derived locally).

Author: mtdowling