servo: Merge #14244 - Use build script to generate binding files (from upsuper:buildtime-bindgen); r=emilio

<!-- Please describe your changes on the following line: -->
This is a WIP patch to make rust-bindgen a build time dependency of style component for stylo. This should make things more automatic. I convert majority of `regen.py` to `build_gecko.rs`.

It currently doesn't work on Windows, because of rust-lang/rust-bindgen#271 (it works when I fix the generated file manually, though). I haven't tested other platforms.

It would break servo's CI for geckolib, because:
1. it needs libclang (which is probably easy to solve)
2. it needs `MOZ_OBJDIR` to be passed in so that it can generate bindings

Manishearth emilio Do you agree with this change? Do you have suggestion for the issues above?

---
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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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Source-Repo: https://github.com/servo/servo
Source-Revision: 872ec89a9c546eb05246b5047aabfc032d140eff

UltraBlame original commit: 78635ebabfc9e63971829a549388d1b61a75a503

Author: marco-c

servo/Cargo.lock

@@ -80,6 +80,16 @@ rayon
 unstable
 "
 ]
+bindgen
+=
+[
+"
+libbindgen
+"
+"
+regex
+"
+]
 servo
 =
 [
@@ -538,6 +548,29 @@ build
 -
 dependencies
 ]
+lazy_static
+=
+"
+0
+.
+2
+"
+libbindgen
+=
+{
+version
+=
+"
+0
+.
+1
+.
+1
+"
+optional
+=
+true
+}
 phf_codegen
 =
 "
@@ -547,6 +580,20 @@ phf_codegen
 .
 20
 "
+regex
+=
+{
+version
+=
+"
+0
+.
+1
+"
+optional
+=
+true
+}
 walkdir
 =
 "

servo/components/style/Cargo.toml

@@ -69,53 +69,3 @@ the
 Rust
 bindings
 .
-#
-#
-regen
-.
-sh
-This
-will
-regenerate
-the
-bindings
-for
-the
-ServoBindings
-.
-h
-file
-in
-your
-gecko
-build
-.
-The
-generated
-bindings
-live
-in
-components
-/
-style
-/
-gecko_bindings
-/
-bindings
-.
-rs
-.
-For
-structs
-the
-bindings
-are
-in
-components
-/
-style
-/
-gecko_bindings
-/
-structs_
-*