Started splitting the preamble.

The semantic commands are now defined in a package called semtex
(semantic tex). As its name indicates, it will blow any normal LaTeX
document to smithereens, as it redefines a lot of usual commands.
Catch up with master.
Catch up with master.
Rebuilt.
Catch up with master.
Refactoring.
Fixed typos before the lecture.
Commit before merging split-preamble.
Merged split-preamble.
First draft of the lecture notes.