Reduce keyword list to something more reasonable, hopefully "finished"

[graydon]

There are still lots of vestigial keywords. We should get this as close to a fixed-set for good as we can.

[graydon]

In particular, again becomes loop (executive fiat; writing again has been a "wince-every-time" experience), remove as many as possible of module, import, export, check, new, owned, send, drop, static, of, with and to, and turn log, assert and fail into built-in macros (that is: log! and assert! and fail!) that call intrinsics that, at present, do what the statements do. And make all remaining "contextual" keywords into normal (restricted everywhere) keywords.

[brson]

Status update: unsafe is the only non-strict keyword now, and I still expect that to be fixed in 0.4. export is still in and I don't expect to be able to remove it. assert, log and fail are still in and I am not planning on converting them to macros for 0.4.

[graydon]

I'll have a go at export today and tomorrow, see if it can go too.

[brson]

unsafe is a real keyword now. There are no more 'restricted' keywords.

[bstrie]

Is const likely to go from keyword to trait?

[brson]

The fate of const is uncertain still. Closing this. Will open a new one for the macro conversions.