Move build -> tools/bazel

Fixes https://github.com/dtolnay/cxx/issues/45.
diff --git a/BUILD b/BUILD
index 5d463b2..0405df1 100644
--- a/BUILD
+++ b/BUILD
@@ -1,4 +1,4 @@
-load("//:build/rust.bzl", "rust_binary", "rust_library")
+load("//tools/bazel:rust.bzl", "rust_binary", "rust_library")
 
 rust_library(
     name = "cxx",
diff --git a/demo-rs/BUILD b/demo-rs/BUILD
index 674b3d9..389ee58 100644
--- a/demo-rs/BUILD
+++ b/demo-rs/BUILD
@@ -1,4 +1,4 @@
-load("//:build/rust.bzl", "rust_binary", "rust_library")
+load("//tools/bazel:rust.bzl", "rust_binary", "rust_library")
 
 rust_binary(
     name = "demo-rs",
diff --git a/tests/BUILD b/tests/BUILD
index a88cde5..16825f6 100644
--- a/tests/BUILD
+++ b/tests/BUILD
@@ -1,4 +1,4 @@
-load("//:build/rust.bzl", "rust_library", "rust_test")
+load("//tools/bazel:rust.bzl", "rust_library", "rust_test")
 
 rust_test(
     name = "test",
diff --git a/third-party/BUILD b/third-party/BUILD
index e2f6d76..9af57d1 100644
--- a/third-party/BUILD
+++ b/third-party/BUILD
@@ -1,5 +1,5 @@
 load(
-    "//:build/rust.bzl",
+    "//tools/bazel:rust.bzl",
     rust_binary = "third_party_rust_binary",
     rust_library = "third_party_rust_library",
 )
diff --git a/tools/bazel/BUILD b/tools/bazel/BUILD
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/tools/bazel/BUILD
diff --git a/build/rust.bzl b/tools/bazel/rust.bzl
similarity index 100%
rename from build/rust.bzl
rename to tools/bazel/rust.bzl