Merge #689: Remove "except in benchmarks" exception for fp math

bde2a32286 Convert bench.h to fixed-point math (Wladimir J. van der Laan)

Pull request description:

  Convert `bench.h` to fixed-point math, removing all use of float math from the repository:

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

  It turned out to be a little trickier than I expected because of formatting and rounding. But, output should be the same before and after.

  I used the following to test the number formatting: https://gist.github.com/laanwj/f971bfbe018e39c19677a21ff954d0c7

ACKs for top commit:
  real-or-random:
    ACK bde2a32286 I've read the code in detail and I've tested it. I haven't explicitly tested the formatting function with known/hardcoded inputs.

Tree-SHA512: 41ab6024b88c65a4b194272097c70d527bedb396dc7ab9d3d93165f1a19d31092798370f66399443a8d5393d0a6dcf5825679de5a325550865cfdef3586bf64c

README.md

@@ -23,7 +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.
+  * No use of floating types.
   * 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).