Add a cabal file for benchmarks

When the installed hashable and the in-place hashable are out of
step, it's really hard to get a benchmark build going using GHC 7.6
without using cabal. Hard enough that I gave up and did this instead.

Author: bos

.hgignore

@@ -1,4 +1,4 @@
-^dist$
+^(?:dist|benchmarks/dist)$
 ^tests/benchmarks/.*\.txt$
 ^tests/(?:\.hpc|bm|qc|qc-hpc|stdio-hpc|text/test)$
 \.(?:aux|eventlog|h[ip]|log|[oa]|orig|prof|ps|rej|swp)$

benchmarks/hashable-benchmarks.cabal

@@ -0,0 +1,20 @@
+name:       hashable-benchmarks
+version:    0
+build-type: Simple
+
+cabal-version: >=1.2
+
+executable hashable-benchmarks
+  ghc-options: -Wall -O2
+  c-sources: ../cbits/hashByteString.c
+  hs-source-dirs: .. .
+  main-is: Benchmarks.hs
+  other-modules:
+    Data.Hashable
+    Data.Hashable.SipHash
+  build-depends:
+    base,
+    bytestring,
+    criterion,
+    ghc-prim,
+    text