PdefValidator

Checks parsed .pdef files before anything is generated from them.

Every problem caught here would otherwise turn up later and worse: as a scalac error pointing into generated code, or - for a field declared twice - as no error at all, just a codec one field short.

The files are checked as one unit because they are generated into one package: a type declared in one file is in scope in all of them, and a name declared twice collides whichever files the two declarations are in.