Export watch_file.

Export watch_file, as this is the easiest API entry point between FileMonitor
and PollingFileMonitor.

Fix _uv_hook_fseventscb such that the condition is notified even if no callback
is in the monitor. Also fix the value sent to notify() to match the expected
return value in wait().

Fix watch_poll default callback.
Document watch_file method.
Add a test for wait(FileMonitor).
Unexport new UV_ constants.
WIP: File watching event refactor.
Re-passing tests.
Refactor events with integer flag fields and accessors.
Fix logic bug in FDWatcher wait()
Merge branch 'watch-refactor' of https://github.com/blakejohnson/julia into jb/watch-refactor
my version of the file watching API refactor
Added helpefull message for DomainError

Some functions throw DomainError for Real arguments because of type
stability and that the correct answer is complex. This patch add a ugly
hack, inspecting the bactrace, to give newcommers a helpefull hint. It
also document some of the functions that throws DomainError for negative
Real arguments.
a couple efficiency tweaks

there were 2 places where more ASTs can be compressed
clean up some unnecessary objects in LambdaStaticData.tfunc
improve round(BigFloat, digits)

this function is supposed to round to n base 10 digits, but instead did:

julia> round(big(2.1),2)
2.094e+00 with 7 bits of precision

which I don't believe is useful.
eliminate more unnecessary objects that were hanging around the system image
close #4252
Fix typo: user_home() -> user_homedir()
Fix #4284
Fix #4284 for slices, too
WINDOWS: fix PATH
Make it possible to pass ncv to eigs.