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

Now, only cells that have been used are cleared. This is achieved by
using a "occupied" memo on the Row itself. The value, `occ`, is updated
wherever the Row is accessed mutably, and it's cleared to zero in
Row::reset.

The tests for grid scroll_up and scroll_down were updated to include a
test on the value `occ` and slightly refactored, but are otherwise
equivalent to the previous implementation of those tests.

Because of the change to the `Row` struct, the ref tests were updated so
Deserialization keeps working as expected.

Some tests are still not passing, though.

A migration script was added to migrate serialized grids from
pre-scrollback to the current format. The script is included with this
commit for completeness, posterity, and as an example to be used in the
future.

A few tests in grid/tests.rs were removed due to becoming irrelevant.

Padding can be configured by using the `padding` field in the config
file, like so:

    padding:
      x: 2
      y: 2

which would result in a 2px padding within each side of the window.

ANSI escape sequences like `\x1b[48;5;10m` were not supported until now.
Specifically, the second attribute, 5, says that the following attribute
is a color index.

The ref tests were updated since `enum Color` variants changed.

Ref tests use a recording of the terminal protocol and a serialization
of the grid state to check that the parsing and action handling systems
produce the correct result. Ref tests may be recorded by running
alacritty with `--ref-test` and closing the terminal by using the window
"X" button. At that point, the recording is fully written to disk, and a
serialization of important state is recorded. Those files should be
moved to an appropriate folder in the `tests/ref/` tree, and the
`ref_test!` macro invocation should be updated accordingly.

A couple of changes were necessary to make this work:

* Ref tests shouldn't create a pty; the pty was refactored out of the
  `Term` type.
* Repeatable lines/cols were needed; on startup, the terminal is resized
* by default to 80x24 though that may be changed by passing
  `--dimensions w h`.
* Calculating window size based on desired rows/columns and font metrics
  required making load_font callable multiple times.
* Refactor types into library crate so they may be imported in an
  integration test.
* A whole bunch of types needed symmetric serialization and
  deserialization. Mostly this was just adding derives, but the custom
  deserialization of Rgb had to change to a deserialize_with function.

This initially adds one ref test as a sanity check, and more will be
added in subsequent commits. This initial ref tests just starts the
terminal and runs `ll`.