src/module-impl.lisp: Eliminate `define-fragment'.

src/module-impl.lisp: Eliminate `define-fragment'.

It had a severely deficient interface: it could only define fragments
with constant contents.  Moreover, although it made an effort to accept
a string-literal as its text, it ended up in a tangle because
`code-fragment-item' insists on its `fragment' slot actually containing
a code fragment, and nothing does the conversion.

Rather than fix these bugs, I noticed that nothing actually used it at
all, so removing it completely seemed like a better option.