Add comparative benchmarks with Taffy 0.3

[nicoburns]

Objective

Allow us to determine how performance has changed since the last major Taffy version.

Changes made

• Depend on Taffy 0.3 (as well as current Taffy) in the benchmarks create
• Implement BuildTree trait for Taffy 0.3
• Add a taffy03 feature flag which runs benchmarks against Taffy 0.3
• Simplify benchmarking code with macros

Context

Since we refactored the benchmarks in #472, simply checking out the old branch no longer gives a useful comparison.