message: Make no-float policy explicit
  Summary:
  We don't want floating types for various reasons, e.g.,
  - Their representation and often their behavior is implementation-defined.
  - Many targets don't support them.
  This is a backport of libsecp256k1 PR684
  Test Plan:
  ninja check-secp256k1
  Reviewers: #bitcoin_abc, Fabien
diff: @@ -24,6 +24,7 @@ Implementation details
  * Extensive testing infrastructure.
  * Structured to facilitate review and analysis.
  * Intended to be portable to any system with a C89 compiler and uint64_t support.
  + * No use of floating types, except in benchmarks.
  * Expose only higher level interfaces to minimize the API surface and improve application security. ("Be difficult to use insecurely.")
  * Field operations
  * Optimized implementation of arithmetic modulo the curve's field size (2^256 - 0x1000003D1).
  