message: Note intention of timing sidechannel freeness.
  Summary:
  Resolves
  This is a backport of secp256k1 PR630
  Test Plan: \o/
  Reviewers: #bitcoin_abc, Fabien
diff: @@ -46,8 +46,10 @@ Implementation details
  * Optionally (off by default) use secp256k1's efficiently-computable endomorphism to split the P multiplicand into 2 half-sized ones.
  * Point multiplication for signing
  * Use a precomputed table of multiples of powers of 16 multiplied with the generator, so general multiplication becomes a series of additions.
  + * Intended to be completely free of timing sidechannels for secret-key operations (on reasonable hardware/toolchains)
  * Access the table with branch-free conditional moves so memory access is uniform.
  * No data-dependent branches
  + * Optional runtime blinding which attempts to frustrate differential power analysis.
  * The precomputed tables add and eventually subtract points for which no known scalar (private key) is known, preventing even an attacker with control over the private key used to control the data internally.
  Build steps
  