rust-filepatterns: also normalize RelPath These patterns should be normalized too; this at least does no harm, and is necessary for proper status support.

rust: simplify pattern file parsing Previously we kept the pattern syntax as a &[u8] until the last possible moment, which meant we had to handle potential errors when parsing that &[u8]. But such errors could never actually occur, given the structure of the code. Now we parse it eagerly (in two places) and pass it around as a PatternSyntax, so we can delete some error handling code. parse_one_pattern is in particular useful for parsing patterns passed on the command line, as we'll support later in this series.

rust-matchers: better support file_set in IntersectionMatcher Previously, if both of the input matchers had non-exact file sets, IntersectionMatcher would simply have an empty fileset. Now, we duplicate Python behavior: we *union* the filesets of the matchers in that case. This makes some sense, because without exact input file sets, we can't do an exact intersection and must over-approximate. Concretely, this is necessary because the file_set is the list of files that were explicitly listed by the user, and we want to provide explicit errors for all such files, from both matchers.