Purge libextra and the test suite of all calls directly into the runtime

[bstrie]

A prerequisite for #3361.

Calls into the runtime look like rustrt::foo().

$ git grep rustrt -- src/libextra/ | wc -l
37

$ git grep rustrt -- src/test/ | wc -l
69

[msullivan]

Current status

sully@anansi:~/src/rust2 [master]$ git grep rustrt -- src/libextra/ | wc -l
28
sully@anansi:~/src/rust2 [master]$ git grep rustrt -- src/test/ | wc -l
73

Going in the right direction in libextra, wrong direction in the test suites.

The uses in libextra are in dbg.rs (what is that for? is it still used?), flate.rs, rl.rs, and time.rs.

[msullivan]

Related to #4812.

[alexcrichton]

Turns out this wasn't a prereq for #3361 because I cheated and linked librustrt to libextra. This isn't in a bad situation at all today, and everything is statically linked as you would expect, so this is no longer an issue.