The crate containing the entry point is called alacritty, and the crate
containing everything else is called alacritty_terminal.

The ConPTY had a cryptic 'assertion failed: success' error whenever
spawning the shell failed.

This changes the ConPTY assertion to a human-readable error which
clearly states that the file could not be found.

The WinPTY backend has not been fixed yet.

This fixes #2016.

If a resize event is identical to the current size, it is no longer
propagated but the resize is discarded immediately.

To further prevent resizes when not necessary, the list of monitors is
enumerated and the DPR of the first display is assumed to be the target
DPR.

This allows spawning a window with dimensions when the config has
columns and lines specified and the window only needs to be resized if
the estimated DPR is not correct.

Fixes #1825.
Fixes #204.

This removes the the signal handling machinery in tty::unix, and
replaces it with functionality from signal-hook, which should be more
robust. Signals caught by signal-hook wake up the existing I/O event
loop, which then delegates back to the PTY to handle them.

In particular, this allows `SIGCHLD` (i.e. child process exits) to shut
down the terminal promptly, instead of sometimes leaving the window
lingering.

Fixes #915.
Fixes #1276.
Fixes #1313.

As a side effect, this fixes a very rare bug on Linux, where a `read`
from the PTY on the master side would sometimes "fail" with `EIO` if the
child closed the client side at a particular moment. This was subject to
a race condition, and was very difficult to trigger in practice.

In cases where the Alacritty process had invalid std handles then
the ConPTY subprocess would fail to spawn. By setting appropriate
flags we prevent these handles from being passed to the ConPTY
subprocess.