Validate builders

[io7m]

Right now, builders allow creating fonts that are structurally sound but still considered invalid by some implementations. In particular, the spec contains text such as:

The zone’s wInstGenNdx points to the first generator for that zone. Unless the zone is a global zone, the last generator in the list is a “sampleID” generator, whose value is a pointer to the sample associated with that zone. If a “key range” generator exists for the zone, it is always the first generator in the list for that zone. If a “velocity range” generator exists for the zone, it will only be preceded by a key range generator. If any generators follow a sampleID generator, they will be ignored.

The builder will happily let you insert generators in any order (it'll use the exact order you specify).