Add state machine utility

Separated out from #5670

Author: jasnell

src/workerd/api/BUILD.bazel

@@ -248,6 +248,7 @@ wd_cc_library(
     visibility = ["//visibility:public"],
     deps = [
         "//src/workerd/io",
+        "//src/workerd/util:state-machine",
         "@nbytes",
     ],
 )

src/workerd/io/BUILD.bazel

@@ -116,6 +116,7 @@ wd_cc_library(
         "//src/workerd/util:ring-buffer",
         "//src/workerd/util:small-set",
         "//src/workerd/util:sqlite",
+        "//src/workerd/util:state-machine",
         "//src/workerd/util:strong-bool",
         "@capnp-cpp//src/capnp:capnp-rpc",
         "@capnp-cpp//src/capnp/compat:http-over-capnp",

src/workerd/util/BUILD.bazel

@@ -387,3 +387,15 @@ kj_test(
     src = "small-set-test.c++",
     deps = [":small-set"],
 )
+
+wd_cc_library(
+    name = "state-machine",
+    hdrs = ["state-machine.h"],
+    visibility = ["//visibility:public"],
+    deps = ["@capnp-cpp//src/kj"],
+)
+
+kj_test(
+    src = "state-machine-test.c++",
+    deps = [":state-machine"],
+)