src/: Add commentary to the generated code.

src/: Add commentary to the generated code.

Introduce a new `banner' instruction whose purpose is to act as section
heading in the code: a blank line is left (except at the head of a
block, hence the earlier expansion of the `format' string) and the
banner text written as a beautifully formatted comment.

The new `format' string, in `format-banner-comment', is just about
interesting enough to make up for the loss of the `block' printer.