Have simple decode methods just return values

[tbkka]

As discussed in PR #243:

https://github.com/apple/swift-protobuf/pull/243/files/e397b41fa359a4aa7f6fec465b4a2d357b661d65#diff-2cc7a2fc1136ffc3ff613b97a60aa72cR63

We should be able to simplify the Decoder protocol by having the simple decode methods (singular fields of primitive types) return a value instead of accepting the field as an inout parameter. For example, the current

decodeSingularInt32Field(value: inout Int32?)
decodeSingularInt32Field(value: inout Int32)

would collapse to a single

decodeSingularInt32Field() -> Int32?

The discussion referenced above covers some of the touchy points:

• JSON does handle null differently for optional-typed and non-optional-typed fields. Optional fields get set to nil, the non-optional ones get set to zero/false/""/etc. Having the decode method return an optional should cover this.
• This would still allow FieldMask to use the decoder API to clear fields.
• We would no longer support reading fields selectively via the decoder API (no current code uses this, but it was possible before and now it won't be).

[thomasvl]

@tbkka should this be in the milestone also?

[tbkka]

Looks like this change makes 100 int64 about 15% slower -- apparently returning an Optional is measurably more expensive than assigning to an inout parameter. (This holds for both proto2 and proto3, interestingly.)

Based on that, I think I'll leave the code with separate decode methods for optional and non-optional field types.

[tbkka]

I'm going to close this -- the current code is a little more verbose, but it's consistent and a little faster so I think it's preferable.