Convert bench.h to fixed-point math

Summary:

• Use 64-bit integer microsecond timestamps
• Use fixed-point math for formatting numbers

Then, remove "except in benchmarks" exception from README.md.

This is a backport of libsecp256k1 PR689

Test Plan:

ninja bench-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5378