Upgrade rust/crates/regex to 1.4.5

Test: make
Change-Id: I1d9343bd9712ddd57023af2c5d248993a2c31088
diff --git a/src/backtrack.rs b/src/backtrack.rs
index 2eaeb72..6100c17 100644
--- a/src/backtrack.rs
+++ b/src/backtrack.rs
@@ -115,8 +115,8 @@
         // Then we reset all existing allocated space to 0.
         // Finally, we request more space if we need it.
         //
-        // This is all a little circuitous, but doing this unsafely
-        // doesn't seem to have a measurable impact on performance.
+        // This is all a little circuitous, but doing this using unchecked
+        // operations doesn't seem to have a measurable impact on performance.
         // (Probably because backtracking is limited to such small
         // inputs/regexes in the first place.)
         let visited_len =