Re-make time struct fields public

These are supposed to be raw C-like structs mirroring time.h's struct tm
and struct timespec.
auto merge of #10034 : sfackler/rust/time, r=alexcrichton

These are supposed to be raw C-like structs mirroring time.h's struct tm
and struct timespec.
auto merge of #10022 : hatahet/rust/master, r=alexcrichton

Fixes #9958