benchmarks: Add subdirectory Benchmarks/ package

[simonjbeaumont]

Motivation

We'd like to be able to benchmark various parts of the runtime library to make targeted and measurable performance improvements.

Modifications

• Add a subdirectory directory package with the package-benchmark plugin.
• Add a benchmark for ISO8601DateTranscoder.encode(_:).
• Add temporary benchmarks of Foundation ISO8601 date encoding APIs to guide improvements to ISO8601DateTranscoder.

Result

• Can now run benchmarks using swift package --package-path Benchmarks/ benchmark.

Test Plan

There is no CI for this currently. That can follow, but we wanted the package plugin to unblock incoming updates to the date transcoder.