flesh out the syntactic extension system

[graydon]

A very basic version of the syntactic extension system is working but only supports a test extension "#shell". Add a few more to see how it feels (#if-#else-#endif, #include, #env, #file, #line, #pos, possibly #re, #html, #bit -matching patterns, maybe a simple macro system?)

For this to really work long-term we need to wait on the self-hosting compiler, but an experiment or two would be useful.

[graydon]

This has been extended to cover a printf-like #fmt extension in rustboot. Shifted to rustc, as it'll have to happen there too.

[graydon]

This also requires some work in the dynamic-linkage department, to load external extensions.

[graydon]

This is as done as it's going to get for first release after pauls' heroic macro system addition over the summer. Punting to "future".

[graydon]

This is too vague to ever close on its own. Open sub-bugs if interested in specific syntax extensions.