[threeten-dev] DateTimeBuilder

[Stephen Colebourne]

I've made the first set of changes to DateTimeBuilder. I pushed them
directly as they should be pretty uncontroversial.

They change the public API of DateTimeFormatter to only refer to
TemporalAccessor, rather than DateTimeBuilder. This mostly involved
test changes.

I also ensured that parsing a weird string with the same field twice
(like the month twice) will report a problem at the parsing stage
rather than at the resolving stage.

I intend to continue work on the TemporalField API to remove builder
there if possible, and to see if there is a better way to code the
builder internally.

Stephen