This adds string serialization for the keycode and modifiers as
used in the config.
We can't simply tell the base types to serialize in this form because
we may serialize and pass those via the mux protocol and the default
derived serializers are more efficient for that purpose.
This allows:
```lua
local wezterm = require 'wezterm'
return {
keys = {
{key="a", mods="ALT", action=wezterm.action{SendKey={key="b"}}}
},
}
```
to parse: previously, wrapping `SendKey` in `wezterm.action` would fail
to round-trip the the `SendKey` and lead to an error loading the
config.
We were previously only remembering the most recently pressed
button, but that's a lossy thing to do.
This commit remembers the button presses so that we can correctly
report all press/release events.
refs: #973
