Return tasks from spawn and spawn_stream

Also, eliminate the background spawning methods. We can spawn futures
on the executor and then spawn those on the app if we need to wait for
the result of running one.

Cargo.lock

@@ -782,6 +782,7 @@ dependencies = [
  "parking_lot",
  "pathfinder_color",
  "pathfinder_geometry",
+ "pin-project",
  "rand 0.8.3",
  "smallvec",
  "smol",
@@ -1076,6 +1077,26 @@ version = "0.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "19b17cddbe7ec3f8bc800887bab5e717348c95ea2ca0b1bf0837fb964dc67099"
 
+[[package]]
+name = "pin-project"
+version = "1.0.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "96fa8ebb90271c4477f144354485b8068bd8f6b78b428b01ba892ca26caf0b63"
+dependencies = [
+ "pin-project-internal",
+]
+
+[[package]]
+name = "pin-project-internal"
+version = "1.0.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "758669ae3558c6f74bd2a18b41f7ac0b5a195aea6639d6a9b5e5d1ad5ba24c0b"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "pin-project-lite"
 version = "0.2.4"