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

Using the `bench` feature, `cargo test` was failing since one of the
benchmarks was running into a debug assertion for attempting to access a
line with an index beyond the grid length.

Since this issue was caused by the `len` property not being serialized
and deserialized, the `#[serde(skip)]` attribute has been changed to
`#[serde(default)]`. The ref-test has been edited to include the correct
grid length for proper deserialization.

This fixes #1604.

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.

Now pass more vttests as well.

Resolves #123
cc #660 - screenshot there exhibited the problem, but issue is not
entirely about the background problem.

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.

Implementing BCE caused a few ref tests to fail erroneously.

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.

The terminal now has a `renderable_cells()` function that returns a
`RenderableCellIter` iterator. This allows reuse of the cell selection
code by multiple renderers, makes it testable, and makes it
independently optimizable.

The render API now takes an `Iterator<Item=IndexedCell>` to support both
the new renderable cells iterator and the `render_string()` method which
generates its own iterator.

The `vim_large_window_scoll` ref test was added here because it provides
a nice large and busy grid to benchmark the cell selection with.