ScanNow() causes the file watch loop to run once, and ensures
all events have been posted before returning. It allows the
caller to "synchronize" with the pending watcher events, so
is useful for batch updating after it is known file events
have probably occurred.

On some file systems, multiple changes to the same file within
the file system's mod time resolution would be missed. This
issue can be ameliorated by triggering `Write` events if a
size change is detected, even if no corresponding mod time
change occurs.

Changes to the file content without a change to the size or
mod time will still go unnoticed.

sometimes when a file/dir which is being watched is
deleted a panic will occur. This appears to be because
the is an assumption that if an error returns true for
`IsNotExist()` that it should be castable to `os.PathError`,
but this evidently is not always the case, causing a panic.

added ScanNow()

fix panic when file/dir is deleted

skip ignored files before callbacks

Preassign map size

Watcher now emits `Write` event for size only change

adding go.mod file

When we close while waiting for the next periodic re-roll or ScanNow we
previously would allow the loop to execute one last time, start a rescan,
and then exit the Start() function when we got a <-w.close.  However,
when this happens and there's a changed files there's a 50% for each file
we'll choose to process the <-evt file event first before the <-w.close!
This is a huge problem if it's no longer safe to do so.

This change simply exits the Start function (safely, closing all the
channels as before) as soon as we receive a <-w.close