Rename Bitwise::population_count to Bitwise::count_ones and add Bitwise::count_zeros

These are inspired by the [functions in the Julia standard library](http://docs.julialang.org/en/release-0.2/stdlib/base/#Base.count_ones).
Forbid use of generics with foreign functions. Closes #10353.
auto merge of #12331 : bjz/rust/count-ones, r=alexcrichton

This is inspired by the [function naming in the Julia standard library](http://docs.julialang.org/en/release-0.2/stdlib/base/#Base.count_ones). It seems like a more self-explanatory name, and is more consistent with the accompanying methods, `leading_zeros` and `trailing_zeros`.
auto merge of #12337 : nick29581/rust/generic_extern, r=alexcrichton

and fix up some formatting.
Fix a deadlock in channels, again.

This deadlock was caused when the channel was closed at just the right time, so
the extra `self.cnt.fetch_add` actually should have preserved the DISCONNECTED
state of the channel. by modifying this the channel entered a state such that
the port would never succeed in dropping.

This also moves the increment of self.steals until after the MAX_STEALS block.
The reason for this is that in 'fn recv()' the steals variable is decremented
immediately after the try_recv(), which could in theory set steals to -1 if it
was previously set to 0 in try_recv().

Closes #12340
auto merge of #12352 : alexcrichton/rust/fix-master, r=brson

This deadlock was caused when the channel was closed at just the right time, so
the extra `self.cnt.fetch_add` actually should have preserved the DISCONNECTED
state of the channel. by modifying this the channel entered a state such that
the port would never succeed in dropping.

This also moves the increment of self.steals until after the MAX_STEALS block.
The reason for this is that in 'fn recv()' the steals variable is decremented
immediately after the try_recv(), which could in theory set steals to -1 if it
was previously set to 0 in try_recv().

Closes #12340