`cbuilder` is now split into `cbuilderbase`, `cbuilderexprs`,
`cbuilderdecls`, with all the struct builder code up to this point going
in `cbuilderdecls`.
Variable builders are added, with local, global and constant variables
implemented, but not threadvars.
A builder for struct (braced) initializers is added. The field names
have to be passed to build each field (so they can be used in `oconstr`
in nifc), but they're not used in the output code if a flag
`orderCompliant` is enabled, which means the initializer list is
generated in order of the built fields. The version which uses the names
on C is not implemented (C99 designated initializers), so this flag has
to be enabled for now.
The struct builders now generate the struct as an inline expression if a
name isn't provided rather than a statement. This means we can now use
`addSimpleStruct` etc for the type of fields, but we can't replace
`addFieldWithStructType` because of `#pragma pack(pop)`.
Doc comments are added to every usable proc but may still not be
sufficient.
