Remove `StringReader::col`.

Remove `StringReader::col`.

It only has a single use, within code handling indented block comments.
We can replace that with the new `FileMap::col_pos()`, which computes
the col position (BytePos instead of CharPos) based on the record of the
last newline char (which we already record).

This is actually an improvement, because
`trim_whitespace_prefix_and_push_line()` was using `col`, which is a
`CharPos`, as a slice index, which is a byte/char confusion.