bench: small tweaks

This adds object files (produced by D compilers) to gitignore, and adds
RE2 to the benchmark compilation script by default.

Author: BurntSushi

bench/.gitignore

@@ -0,0 +1 @@
+*.o

bench/compile

@@ -5,5 +5,5 @@ export RUSTFLAGS="-C target-cpu=native"
 
 exec cargo build \
   --release \
-  --features 're-onig re-pcre1 re-pcre2 re-rust re-rust-bytes re-tcl re-dphobos-dmd re-dphobos-ldc' \
+  --features 're-re2 re-onig re-pcre1 re-pcre2 re-rust re-rust-bytes re-tcl re-dphobos-dmd re-dphobos-ldc' \
   "$@"