Auto merge of #11816 - servo:stable-style, r=nox

Make the style crate (almost) build with a stable compiler

<!-- Please describe your changes on the following line: -->

The bulk of this is adding cargo features to make derived implementations of `heapsize` and `serde` traits optional.

"Almost" because `std::intrinsics::discriminant_value` is currently unstable and doesn’t have any stable replacement that I know of. For now, this PR conditionally replaces it with `unimplemented!()`.

r? @nox

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes <s>fix</s> are part of #11815 (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not <s>require tests</s> *have tests yet* because that requires https://github.com/servo/servo/issues/11806, but I still want to land this before it bitrots. (Tests should check that the build succeeds with a stable compiler.)

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components/servo/Cargo.lock

@@ -2215,7 +2215,6 @@ dependencies = [
  "log 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "matches 0.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "num-traits 0.1.32 (registry+https://github.com/rust-lang/crates.io-index)",
- "plugins 0.0.1",
  "rustc-serialize 0.3.19 (registry+https://github.com/rust-lang/crates.io-index)",
  "selectors 0.6.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde 0.7.10 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -2253,7 +2252,6 @@ dependencies = [
  "euclid 0.6.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "heapsize 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "heapsize_plugin 0.1.5 (registry+https://github.com/rust-lang/crates.io-index)",
- "plugins 0.0.1",
  "rustc-serialize 0.3.19 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde 0.7.10 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_macros 0.7.10 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -2438,7 +2436,6 @@ dependencies = [
  "euclid 0.6.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "getopts 0.2.14 (registry+https://github.com/rust-lang/crates.io-index)",
  "heapsize 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
- "heapsize_plugin 0.1.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "ipc-channel 0.2.3 (git+https://github.com/servo/ipc-channel)",
  "kernel32-sys 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "lazy_static 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -2446,7 +2443,6 @@ dependencies = [
  "log 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "num-traits 0.1.32 (registry+https://github.com/rust-lang/crates.io-index)",
  "num_cpus 0.2.12 (registry+https://github.com/rust-lang/crates.io-index)",
- "plugins 0.0.1",
  "rand 0.3.14 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc-serialize 0.3.19 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde 0.7.10 (registry+https://github.com/rust-lang/crates.io-index)",

components/servo/Cargo.toml

@@ -59,7 +59,7 @@ script_traits = {path = "../script_traits"}
 layout = {path = "../layout"}
 layout_thread = {path = "../layout_thread"}
 gfx = {path = "../gfx"}
-style = {path = "../style"}
+style = {path = "../style", features = ["servo"]}
 canvas = {path = "../canvas"}
 canvas_traits = {path = "../canvas_traits"}
 devtools = {path = "../devtools"}