Better stream.pipe() tracking.

This commit does three things:

1. Uses an exposed counter rather than a hidden array for tracking dest
streams that may have multiple inputs.  This allows for significantly
faster lookups, since the counter can be checked in constant time rather
than searching an array for the dest object.  (A proper O(1) WeakMap
would be better, but that may have to wait for Harmony.)

2. Calls the 'end' event logic when there is an 'error' event on the
source object (and then throws if there are no other error listeners.)
This is important, because otherwise 'error' events would lead to
memory leaks.

3. Clean up the style a bit.  Function Declarations are not allowed
within blocks in ES strict.  Prefer Function Declarations to Function
Expressions, because hoisting allows for more expressive ordering of
logic.

Downside: It adds "_pipeCount" as part of the Stream API.  It'll work
fine if the member is missing, but if anyone tries to use it for some
other purpose, it can mess things up.