Introduce new FixedLenSlice constructor

It is used in the case where a variable-length slice pattern is used to
match on an array of known size. This allows considering only those
entries in the array that are captured by one of the patterns.
As a side-effect, diagnostics improve a bit for those cases.

src/test/ui/pattern/usefulness/match-byte-array-patterns-2.stderr

@@ -1,8 +1,8 @@
-error[E0004]: non-exhaustive patterns: `&[_, _, _, _]` not covered
+error[E0004]: non-exhaustive patterns: `&[..]` not covered
   --> $DIR/match-byte-array-patterns-2.rs:4:11
    |
 LL |     match buf {
-   |           ^^^ pattern `&[_, _, _, _]` not covered
+   |           ^^^ pattern `&[..]` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms