Currently the fields is a list of lists of strings. This will needlessly
consume RAM so lets switch it over to a simple string (which is immutable
and can be stored in flash).
We also replace indices with simple x and y variables. In addition to
avoiding a (temporary) memory allocation this is also easier to use
when looking up in fields.
Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>
e570bf4f26