Add support for serializing trees as JSON

This adds support for serializing the mdast syntax tree as JSON,
with serde, through a feature.

Closes GH-10.
Related-to GH-30.
Closes GH-37.

Author: kyle-mccarthy

.github/workflows/main.yml

@@ -11,8 +11,8 @@ jobs:
         with:
           toolchain: stable
           components: rustfmt, clippy
-      - run: cargo fmt --check && cargo clippy --examples --tests --benches
-      - run: cargo test
+      - run: cargo fmt --check && cargo clippy --examples --tests --benches --all-features
+      - run: cargo test --all-features
   coverage:
     runs-on: ubuntu-latest
     steps:

Cargo.toml

@@ -17,9 +17,15 @@ name = "bench"
 path = "benches/bench.rs"
 harness = false
 
+[features]
+default = ["json"]
+json = ["dep:serde", "dep:serde_json"]
+
 [dependencies]
 log = "0.4"
 unicode-id = { version = "0.3", features = ["no_std"] }
+serde = { version = "1.0", optional = true }
+serde_json = { version = "1.0", optional = true }
 
 [dev-dependencies]
 env_logger = "0.10"