Performance: formatting float/double for json and text formats

[tbkka]

Writing float and double values to JSON or text format is significantly slower than it should be.

The original code used String(_: Double) to convert doubles to String, and then converted the String to UTF8 to append to the output. PR #391 changes this to directly invoke the C library strtod and vsnprintf to parse and format doubles, respectively.

The direct use of strtod worked very well, but vsnprintf requires using Swift's CVarArg facility; this involves creating intermediate objects that we usually manage to avoid. If we could avoid using varargs in this way, we should see a sizable performance improvement. Unfortunately, the standard C library does not expose a function for formatting float/double that doesn't use varargs.

[tbkka]

Related: The other problem with String(_: Double) is that it's not exact. (Google's conformance test checks for this.) See https://bugs.swift.org/browse/SR-106 for more discussion.

[tbkka]

Hmmm.... Looks like the exactness problem was partially addressed in swiftlang/swift#348 (Double.debugDescription is now exact; Double.description behaves the same as before).

[tbkka]

Same as #752, resolved by #882.