Add no_std test example and update alloc gating

Update codegen to make `bytes` imports conditional on the `alloc`
feature and add a test example for the `thumbv7em-none-eabihf`
target.

Cargo.lock

@@ -310,6 +310,12 @@ dependencies = [
  "itertools",
 ]
 
+[[package]]
+name = "critical-section"
+version = "1.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "790eea4361631c5e7d22598ecd5723ff611904e3344ce8720784c93e3d83d40b"
+
 [[package]]
 name = "crossbeam-deque"
 version = "0.8.6"
@@ -347,6 +353,16 @@ version = "1.15.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "48c757948c5ede0e46177b7add2e67155f70e33c07fea8284df6576da70b3719"
 
+[[package]]
+name = "embedded-alloc"
+version = "0.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ddae17915accbac2cfbc64ea0ae6e3b330e6ea124ba108dada63646fd3c6f815"
+dependencies = [
+ "critical-section",
+ "linked_list_allocator",
+]
+
 [[package]]
 name = "env_filter"
 version = "1.0.1"
@@ -776,6 +792,12 @@ version = "0.2.186"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "68ab91017fe16c622486840e4c83c9a37afeff978bd239b5293d61ece587de66"
 
+[[package]]
+name = "linked_list_allocator"
+version = "0.10.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2b23ac50abb8261cb38c6e2a7192d3302e0836dac1628f6a93b82b4fad185897"
+
 [[package]]
 name = "linux-raw-sys"
 version = "0.12.1"
@@ -836,6 +858,8 @@ checksum = "1d87ecb2933e8aeadb3e3a02b828fed80a7528047e68b4f424523a0981a3a084"
 name = "no_std_test"
 version = "0.1.0"
 dependencies = [
+ "bytes",
+ "embedded-alloc",
  "roto-runtime",
 ]
 
@@ -1201,6 +1225,7 @@ dependencies = [
 name = "roto-tonic"
 version = "0.1.0"
 dependencies = [
+ "async-trait",
  "bytes",
  "futures-util",
  "http",