style(prost-build): Consolidate field data into struct (#1017)

* prost-build: consolidate message field data

When massaging field data in CodeGenerator::append_message,
move it into lists of Field and OneofField structs so that later
generation passes can operate on the data with less code duplication.

Subsidiary append_* methods are changed to take references to these
structs rather than moved data, as generation of lexical tokens
does not actually consume any owned data, and we will need more
passes over the same field lists for the upcoming builder code.

* prost-build: compute field tags in place

* prost-build: address comments on reuse of Field

Make rust_field into a method computing the name on the fly.
In OneofField, make the vector of fields to have Field members.
Don't play reference renaming tricks with field.descriptor.