Rename (float,double)-(le,be) to (float,double)(le,be).

It's what FFmpeg seems to call them, so let's go with that.
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), and a number of modules
defining certain descriptor types and descriptors that come with the
module.