Move FloatingPoint.swift out of .gyb.

Rework Self._convert from integer, by making it require RawSignificand: FixedWidthInteger
This requirement should have always been there, and the existing implementaiton wouldn't
have actually worked correctly without it. Making it explcit makes the implementation quite
a bit simpler, which is nice.

Add fast-path conversion that will catch all concrete integer types without needing to be
a concrete implementation itself. Room for further improvement, but good start.

Author: stephentyrone

stdlib/public/core/CMakeLists.txt

@@ -66,7 +66,7 @@ set(SWIFTLIB_ESSENTIAL
   FixedArray.swift.gyb
   FlatMap.swift
   Flatten.swift
-  FloatingPoint.swift.gyb
+  FloatingPoint.swift
   FloatingPointParsing.swift.gyb
   FloatingPointTypes.swift.gyb
   Hashable.swift