Change from double to float for floating point values

[mikebmcl]

double is overkill presently and would non-trivially degrade performance on many platforms. There are times when double is the right choice (see, e.g., Random ASCII - Don't Store That in a Float), but none of the usages in this API appear to meet that criteria. Further, using double rather than float would hurt performance both when using modern GPUs for hardware acceleration and when running on low power, non-freestanding devices.