pr: don't convert to String when storing lines to print

[venoosoo]

Changed FileLine.line_content from String to Vec<u8> to avoid
unnecessary byte → String → byte conversions.

Changes:

• FileLine.line_content is now Vec<u8>
• apply_expand_tab now takes &mut Vec<u8> instead of &mut String
• from_buf no longer needs to return Result since UTF-8 validation
  is no longer needed
• get_pages also no longer needs to return Result as a cascade effect
• get_line_for_printing uses String::from_utf8_lossy only at the
  formatting stage

Note: test_dd::test_iso8859_1_case_conversion fails but is pre-existing
and unrelated to this change.

Fixes #10333

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.

[venoosoo]

the error on the test seems infrastructure issue unrelated to this change — the VM fails at SSH connection. Style and lint checks passed

[cakebaker]

While this is an improvement to the previous code, it's not entirely correct. GNU pr also prints non-UTF8 characters.

You can see the difference with a tool like hexdump:

$ printf $'\xFFfoo' | cargo run -q pr | hexdump -C
[snipped]
*
00000040  20 20 20 20 50 61 67 65  20 31 0a 0a 0a ef bf bd  |    Page 1......|
00000050  66 6f 6f 0a 0a 0a 0a 0a  0a 0a 0a 0a 0a 0a 0a 0a  |foo.............|
00000060  0a 0a 0a 0a 0a 0a 0a 0a  0a 0a 0a 0a 0a 0a 0a 0a  |................|
*
00000090
$ printf $'\xFFfoo' | pr | hexdump -C
[snipped]
*
00000040  20 20 20 20 50 61 67 65  20 31 0a 0a 0a ff 66 6f  |    Page 1....fo|
00000050  6f 0a 0a 0a 0a 0a 0a 0a  0a 0a 0a 0a 0a 0a 0a 0a  |o...............|
00000060  0a 0a 0a 0a 0a 0a 0a 0a  0a 0a 0a 0a 0a 0a 0a 0a  |................|
*
00000080  0a 0a 0a 0a 0a 0a 0a 0a  0a 0a 0a 0a 0a 0a        |..............|
0000008e

My suggestion is to add a todo that support for non-UTF8 is not implemented yet.

[cakebaker]

A detail: this part of the comment is no longer valid and can be removed.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/io-errors. tests/misc/io-errors is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)

[cakebaker]

Thanks for your PR!