Switch to UnicodeScalarView for performance
Switch to UnicodeScalarView for performance

This seems to improve decoding speed by as much as 20x
(In one run on my laptop, the decode perf test went
from ~7s to ~.3s for 1000 iterations.)

This could probably be sped up further:  the identifiers and
strings could likely return slices of the original instead of
cosnstructing new strings; it might be faster to parse
UTF-8 directly instead of Unicode scalars.

cburrows/swift-protobuf,apple/swift-protobuf,tbkka/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf,cburrows/swift-protobuf,cburrows/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,tbkka/swift-protobuf,allevato/swift-protobuf,tbkka/swift-protobuf,cburrows/swift-protobuf,tbkka/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf,allevato/swift-protobuf,thomasvl/swift-protobuf,allevato/swift-protobuf,thomasvl/swift-protobuf,cburrows/swift-protobuf,allevato/swift-protobuf,allevato/swift-protobuf,thomasvl/swift-protobuf,allevato/swift-protobuf,apple/swift-protobuf,tbkka/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,cburrows/swift-protobuf,allevato/swift-protobuf,thomasvl/swift-protobuf