Fix example feature dependencies

Turns out that all features rely on std, not only sys, in some capacity.
Most of the failures are from parsing of address which is implemented in
terms of std (see #26).

ethox/Cargo.toml

@@ -24,18 +24,23 @@ sys = ["libc"]
 [dev-dependencies]
 structopt = { version = "0.2", default-features = false }
 
+# All examples rely on sys and std for some reason.
+#
+# sys: Required to open the underlying sockets.
+# std: Most due to argument parsing (which is done in std) but also to use
+#   stderr and other io portions
 [[example]]
 name = "debug_tap"
-required-features = ["alloc", "sys"]
+required-features = ["alloc", "sys", "std"]
 
 [[example]]
 name = "ping_tap"
-required-features = ["alloc", "sys"]
+required-features = ["alloc", "sys", "std"]
 
 [[example]]
 name = "arp_tap"
-required-features = ["alloc", "sys"]
+required-features = ["alloc", "sys", "std"]
 
 [[example]]
 name = "curl"
-required-features = ["alloc", "sys"]
+required-features = ["alloc", "sys", "std"]