bench: Add texray span output

[samuelburnham]

• Adds a texray tracing subscriber to print a timing graph and peak RAM usage per span
• Adds a TEXRAY_PREFIXES env var that tunes which spans are printed. By default just stark/ and bench/ prints from multi-stark, but unset TEXRAY_PREFIXES= to see the spans from Plonky3
• Changes log_blowup to 2 to match the Aiur use case