Re-structure/split the module.

The name "procedural.scm" was already obsolete since a syntactic
version is not considered anymore, so now we have a "base" that
defines the framework itself (bytestructure descriptor types,
bytestructure descriptors, bytestructures), a number of modules
defining certain descriptor types and descriptors that come with the
module, and a "standard" module that imports and re-exports them all
at once, for convenience.