Make a time test less flaky

This test was failing periodically on windows and other platforms, and in
debugging the issue locally I've found that the previous test was failing
at the assertion `ns0 <= ns1`. Upon inspecting the values, the two numbers were
very close to one another, but off by a little bit.

I believe that this is because `precise_time_s` goes from `u64` -> `f64` and
then we go again back to `u64` for the assertion. This conversion is a lossy one
that's not always guaranteed to succeed, so instead I've changed the test to
only compare against u64 instances.
Implemented BufWriter

Filled in the implementations of Writer and Seek for BufWriter. It
raises the io_error condition if a write cannot fit in the buffer.

The Seek implementation for MemWriter, which was incorrectly using
unsigned arithmatic to add signed offsets, has also been replaced.
auto merge of #10451 : zkamsler/rust/buf_writer, r=alexcrichton

I implemented BufWriter. I realize the use of conditions are on their way out for IO, but it does raise a condition if a write will not fit in the buffer for now.

I also replaced the seek code for MemWriter. It was adding the offset as a uint, which is unsound for negative offsets. It only happened to work because unsigned addition performs the same operation with two's complement, and sizeof(uint) <= sizeof(i64) so there was no (lack of) sign extension. I replaced this with computing an offset as an i64 and clamping to zero. I don't expect anyone will have use BufWriter with a byte buffer greater than 2^63 bytes any time soon.

@alexcrichton

Closes #10433
auto merge of #10447 : alexcrichton/rust/flaky-time-test, r=catamorphism

This test was failing periodically on windows and other platforms, and in
debugging the issue locally I've found that the previous test was failing
at the assertion `ns0 <= ns1`. Upon inspecting the values, the two numbers were
very close to one another, but off by a little bit.

I believe that this is because `precise_time_s` goes from `u64` -> `f64` and
then we go again back to `u64` for the assertion. This conversion is a lossy one
that's not always guaranteed to succeed, so instead I've changed the test to
only compare against u64 instances.