Improve GNU compatibility through byte processing

The original Rust implementation assummed a UTF-8 environment, as is
common in modern systems.  This commit improves GNU sed compatibility
by changing most processing to run on bytes rather than Rust's Unicode
characters and strings. This change also improves the handling of binary
files. The commit adds a simple POSIX-compatible locale configuration
to support UTF-8 processing when specified. While at it, also fix
script-specified file handling to take place through OsString.

The change fixes two failing GNU test suite cases.

Author: dspinellis

README.md

@@ -116,12 +116,19 @@ cargo test
   sequences.
 
 ### Incompatibilities
-* The input is assumed to be valid UTF-8 (this includes 7-bit ASCII).
-  If the input is in another code page, consider converting it through UTF-8
-  in order to avoid errors on invalid UTF-8 sequences and for the correct
-  handling of regular expressions.
-  This _sed_ program can also handle arbitrary byte sequences if no part of the
-  input is treated as string.
+* Similarly to GNU _sed_, input is processed as raw bytes or as valid UTF-8
+  (this includes 7-bit ASCII) based on the locale as specified by the
+  `LC_ALL`, `LC_CTYPE`, and `LANG` environment variables,
+  with the default being byte processing.
+  However, in contrast with GNU _sed_, other locales (e.g. ISO-8859-1)
+  are not supported. If the input is in another code page or encoding
+  and requires locale-specific processing (e.g. ignore/map case,
+  character classes), consider converting it through UTF-8 to ensure
+  the correct handling of locale-specific regular expressions.
+  This _sed_ program can also handle arbitrary byte sequences
+  if no part of the input requires treating it as a Rust String.
+* Back-references aren't supported when input is processed as bytes
+  (`LC_ALL=C`).
 * The command will report an error and fail if duplicate labels are found
   in the script.
   This matches the BSD behavior. The GNU version accepts duplicate labels.