Genesis State Generator + Interop Docs (#3405)

* add main.go

* interop readme

* proper visibility

* standardize and abstract into simpler funcs

* formatting

* no os pkg

* add test

* no panics anywhere, properly and nicely handle errors

* proper comments

* fix broken test

* readme

* comment

* recommend ssz

* install

* tool now works

* README

* build

* readme

* 64 validators

* rem print

beacon-chain/core/state/BUILD.bazel

@@ -7,7 +7,10 @@ go_library(
         "transition.go",
     ],
     importpath = "github.com/prysmaticlabs/prysm/beacon-chain/core/state",
-    visibility = ["//beacon-chain:__subpackages__"],
+    visibility = [
+        "//beacon-chain:__subpackages__",
+        "//tools/genesis-state-gen:__pkg__",
+    ],
     deps = [
         "//beacon-chain/core/blocks:go_default_library",
         "//beacon-chain/core/epoch:go_default_library",