auto merge of #15369 : omasanori/rust/asctime, r=alexcrichton

auto merge of #15369 : omasanori/rust/asctime, r=alexcrichton

In C, `ctime(t)` is equivalent to `asctime(localtime(t))`, so the result should depend on the local timezone. Current `ctime` is compatible with `asctime` in C, not `ctime`.

This commit renames `ctime` to `asctime` and adds `ctime` which converts the time to the local timezone before formatting it.

This commit also fixes the documentation of them. Current documentation of `ctime` says it returns "a string of the current time." However, it actually returns a string of the time represented as `self`, not the time when it is called.