Move SHA256 values of clang and go to revision.bzl. (#36)

This enables better reuse of those revision and sha values.
diff --git a/WORKSPACE b/WORKSPACE
index 7a8a7cb..53ea58e 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -169,34 +169,43 @@
     ],
 )
 
+load(
+    "//third_party/golang:revision.bzl",
+    "GOLANG_REVISION",
+    "GOLANG_SHA256",
+)
+
 # Golang
 http_file(
     name = "golang_release",
-    sha256 = "b5a64335f1490277b585832d1f6c7f8c6c11206cba5cd3f771dcb87b98ad1a33",
-    urls = ["https://storage.googleapis.com/golang/go1.10.linux-amd64.tar.gz"],
+    sha256 = GOLANG_SHA256,
+    urls = ["https://storage.googleapis.com/golang/go" + GOLANG_REVISION + ".linux-amd64.tar.gz"],
 )
 
 load(
     "//third_party/clang:revision.bzl",
     "CLANG_REVISION",
+    "DEBIAN8_CLANG_SHA256",
+    "DEBIAN9_CLANG_SHA256",
+    "UBUNTU16_04_CLANG_SHA256",
 )
 
 # Clang
 http_file(
     name = "debian8_clang_release",
-    sha256 = "588ddbd83c32e7ab87fad58038c3712f6d1b0056f12ea746169ac7ecd33c93bf",
+    sha256 = DEBIAN8_CLANG_SHA256,
     urls = ["https://storage.googleapis.com/clang-builds-stable/clang-debian8/clang_" + CLANG_REVISION + ".tar.gz"],
 )
 
 http_file(
     name = "debian9_clang_release",
-    sha256 = "434bbe2195845e42eb36ad2b88f6aec658d192614d47f71e15b861e09f71b4bd",
+    sha256 = DEBIAN9_CLANG_SHA256,
     urls = ["https://storage.googleapis.com/clang-builds-stable/clang-debian9/clang_" + CLANG_REVISION + ".tar.gz"],
 )
 
 http_file(
     name = "ubuntu16_04_clang_release",
-    sha256 = "48bfc470810d483ffb2e826ed3540efc1da45fa89f416f8efa439242495e6239",
+    sha256 = UBUNTU16_04_CLANG_SHA256,
     urls = ["https://storage.googleapis.com/clang-builds-stable/clang-ubuntu16_04/clang_" + CLANG_REVISION + ".tar.gz"],
 )
 
diff --git a/third_party/clang/revision.bzl b/third_party/clang/revision.bzl
index b3405f7..c6cc608 100644
--- a/third_party/clang/revision.bzl
+++ b/third_party/clang/revision.bzl
@@ -1 +1,4 @@
 CLANG_REVISION="r327695"
+DEBIAN8_CLANG_SHA256="588ddbd83c32e7ab87fad58038c3712f6d1b0056f12ea746169ac7ecd33c93bf"
+DEBIAN9_CLANG_SHA256="434bbe2195845e42eb36ad2b88f6aec658d192614d47f71e15b861e09f71b4bd"
+UBUNTU16_04_CLANG_SHA256="48bfc470810d483ffb2e826ed3540efc1da45fa89f416f8efa439242495e6239"
diff --git a/third_party/golang/revision.bzl b/third_party/golang/revision.bzl
new file mode 100644
index 0000000..0484f05
--- /dev/null
+++ b/third_party/golang/revision.bzl
@@ -0,0 +1,2 @@
+GOLANG_REVISION="1.10"
+GOLANG_SHA256="b5a64335f1490277b585832d1f6c7f8c6c11206cba5cd3f771dcb87b98ad1a33"