[sledge] Compile debug mode with optimization enabled

Summary:
The ~20% hit in compilation time is dominated by the time saved
running the tests. This also makes the memory allocation numbers more
robust to inconsequential changes. Note that `make check` is still
fast.

Note that normally dune passes `-g` to ocamlopt itself, but not if the
build profile includes an `ocamlopt_flags` entry. So add it explicitly
to ensure that compilation includes debug symbols.

(Also remove `-error-style short` that was missed in 9c4f263.)

Reviewed By: jvillard

Differential Revision: D25756570

fbshipit-source-id: 94d18bac2

sledge/dune

@@ -7,20 +7,21 @@
  (debug
   (flags
    (-w +a-4-9-18-30-40-42-44-48@50-66 -strict-formats -strict-sequence
-     -short-paths -bin-annot -keep-locs -keep-docs -opaque))
+     -short-paths -bin-annot -keep-locs -keep-docs -g))
+  (ocamlopt_flags (-O3))
   (env-vars
    (PPX_TRACE_ENABLED 1))
   (inline_tests enabled))
  (trace
   (flags
-   (-w -a -error-style short -noassert -unboxed-types))
+   (-w -a -noassert -unboxed-types -g))
   (ocamlopt_flags (-O3))
   (env-vars
    (PPX_TRACE_ENABLED 1))
   (inline_tests disabled))
  (_ ; release, dev, default,...
   (flags
-   (-w -a -error-style short -noassert -unboxed-types))
+   (-w -a -noassert -unboxed-types))
   (ocamlopt_flags (-O3))
   (env-vars
    (PPX_TRACE_ENABLED 0))