Rename Option swap_unwrap to take_unwrap. 

Fixes Issue #7764

Running `make check` I do get a failure:

    test rt::io::extensions::test::push_bytes ... ok
    rustest rt::comm::test::oneshot_single_thread_send_port_close ... t: task failed at 'Unhandled condition:
     read_error: {kind: OtherIoError, desc: "Placeholder error. You shouldn\'t be seeing this", detail: None}',
     /Users/shout/Projects/rust/src/libstd/condition.rs:50
    /bin/sh: line 1: 35056 Abort trap: 6           x86_64-apple-darwin/stage2/test/stdtest-x86_64-apple-darwin --logfile
     tmp/check-stage2-T-x86_64-apple-darwin-H-x86_64-apple-darwin-std.log
    make: *** [tmp/check-stage2-T-x86_64-apple-darwin-H-x86_64-apple-darwin-std.ok] Error 134