Merge bitcoin-core#684: Make no-float policy explicit

bae1bea Make no-float policy explicit (Tim Ruffing)

Pull request description:

  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.

  Closes bitcoin-core#683.

ACKs for top commit:
  jonasnick:
    ACK bae1bea

Tree-SHA512: e0027d6dda1a3e4b7d146fd3bea04e05473e08e25c0d0730018768be00351dfcf51b87b47b9e27953a21d42e0621433f13cbe55e4c20a7f7086e0191dff607a6

Author: jonasnick

README.md

@@ -23,6 +23,7 @@ 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).