See if there's any benefit to adding more specialized methods to WriteBuffer

[popematt]

         buffer.writeFlexUInt(2)
         buffer.writeFixedIntOrUInt(bits, 2)

This pattern recurs frequently. I wonder if it would be (marginally) faster to have a method variant that does both in one step.

Originally posted by @tgregg in #1133 (comment)

E.g.:

buffer.writeFixedIntWithLengthPrefix(2, bits)

or:

buffer.writeFixedIntWithLengthPrefix2(bits)

Although, we may see some benefit just from having constants for more FlexUInt values. E.g.:

buffer.writeByte(FlexInt.TWO)
buffer.writeFixedInt2(bits)