Loosen Rust crate's tree-sitter dependency (#129)

Nothing in the Rust binding depends on tree-sitter 0.20 in particular;
it works just fine if you haven't updated from 0.19 yet.  (Other
versions might work as well; these are the only two that I tested.)

Cargo.toml

@@ -25,7 +25,7 @@ include = [
 path = "bindings/rust/lib.rs"
 
 [dependencies]
-tree-sitter = "0.19"
+tree-sitter = ">= 0.19, < 0.21"
 
 [build-dependencies]
 cc = "1.0"