Move gecko_bindings and gecko_string_cache into the style crate

components/style/binding_tools/README.md

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+# GeckoLib tools
+
+This directory contains mostly simple tools for working with
+[stylo](https://public.etherpad-mozilla.org/p/stylo).
+
+Some scripts require [multirust](https://github.com/brson/multirust) in order to
+work.
+
+You can see a description of them below.
+
+## `setup_bindgen.sh`
+
+This uses downloads a custom version of bindgen, up to date to generate the
+bindings, and uses the required `llvm38` to build it.
+
+It will also rebuild it if it's already downloaded.
+
+## `regen_bindings.sh`
+
+This will regenerate the bindings for the `ServoBindings.h` file in your gecko
+build (which are in `ports/geckolib/bindings.rs`).
+
+## `regen_style_structs.sh`
+
+This will generate the bindings for Gecko's style structs. Current bindings are
+actually in `ports/geckolib/structs.rs`.