This allow reuse of the implementation of `LineProgram::from` as part
of a more complex transformation.

Operation of `LineProgram::from` is mostly unchanged. The one difference
is that it now uses the string form chosen by `LineString::new`
instead of copying the form of the input.

We now automatically start sequences if needed.

This better matches how the underlying DWARF instructions operate,
and simplifies use of `LineConvert` since we no longer need to
check if a sequence has started.

This also means that `LineConvert::read_row` will work as expected
if there are multiple `DW_LNE_set_address` in a sequence.
`LineConvert::read_sequence` still does not support this.

LineConvert::new already retrieves the compilation name from the
program.