squash all warnings, add RTT support

Cargo.lock

@@ -264,6 +264,7 @@ dependencies = [
  "nb 1.0.0",
  "num-traits",
  "panic-halt",
+ "rtt-target",
  "smoltcp",
  "stm32-eth",
  "stm32f4xx-hal",
@@ -368,6 +369,16 @@ version = "0.6.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ec0be4795e2f6a28069bec0b5ff3e2ac9bafc99e6a9a7dc3547996c5c816922c"
 
+[[package]]
+name = "rtt-target"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "065d6058bb1204f51a562a67209e1817cf714759d5cf845aa45c75fa7b0b9d9b"
+dependencies = [
+ "cortex-m 0.7.6",
+ "ufmt-write",
+]
+
 [[package]]
 name = "rustc_version"
 version = "0.2.3"
@@ -496,6 +507,12 @@ version = "1.15.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "dcf81ac59edc17cc8697ff311e8f5ef2d99fcbd9817b34cec66f90b6c3dfd987"
 
+[[package]]
+name = "ufmt-write"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e87a2ed6b42ec5e28cc3b94c09982969e9227600b2e3dcbc1db927a84c06bd69"
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.5"