Introduce "bundler" feature flag

This commit adds a "bundler" feature flag to the lightningcss crate.

We put the "dashmap" and "rayon" dependencies behind this new flag.

Users that do not need the bundler can now disable the feature to save
on compile time.

Related: #357

Cargo.toml

@@ -26,6 +26,14 @@ name = "lightningcss"
 path = "src/lib.rs"
 crate-type = ["rlib"]
 
+[features]
+default = ["bundler", "grid"]
+browserslist = ["browserslist-rs"]
+bundler = ["dashmap", "rayon"]
+cli = ["clap", "serde_json", "browserslist", "jemallocator"]
+grid = []
+serde = ["smallvec/serde", "cssparser/serde"]
+
 [dependencies]
 serde = { version = "1.0.123", features = ["derive"] }
 cssparser = "0.29.1"
@@ -41,8 +49,8 @@ pathdiff = "0.2.1"
 # CLI deps
 clap = { version = "3.0.6", features = ["derive"], optional = true }
 browserslist-rs = { version = "0.7.0", optional = true }
-rayon = "1.5.1"
-dashmap = "5.0.0"
+rayon = { version = "1.5.1", optional = true }
+dashmap = { version = "5.0.0", optional = true }
 serde_json = { version = "1.0.78", optional = true }
 lightningcss-derive = { version = "1.0.0-alpha.35", path = "./derive" }
 
@@ -56,13 +64,6 @@ assert_fs = "1.0"
 predicates = "2.1"
 serde_json = "1"
 
-[features]
-default = ["grid"]
-browserslist = ["browserslist-rs"]
-cli = ["clap", "serde_json", "browserslist", "jemallocator"]
-grid = []
-serde = ["smallvec/serde", "cssparser/serde"]
-
 [[test]]
 name = "cli_integration_tests"
 path = "tests/cli_integration_tests.rs"

src/lib.rs

@@ -16,6 +16,7 @@
 
 #![deny(missing_docs)]
 
+#[cfg(feature = "bundler")]
 pub mod bundler;
 mod compat;
 mod context;