Move witx program to an example (#243)

This allows crates which depend on `witx` for library support to avoid
pulling in `pretty_env_logger` and `structopt` since those are only used
by the binary program instead of the library itself.

Author: alexcrichton

tools/witx/Cargo.toml

@@ -13,17 +13,13 @@ edition = "2018"
 [lib]
 crate-type=["rlib"]
 
-[[bin]]
-name = "witx"
-path = "src/main.rs"
-
 [dependencies]
-structopt = "0.3"
-wast = { version = "9.0.0", default-features = false }
-thiserror = "1.0"
-log = "0.4"
-pretty_env_logger = "0.3"
 anyhow = "1"
+log = "0.4"
+thiserror = "1.0"
+wast = { version = "9.0.0", default-features = false }
 
 [dev-dependencies]
 diff = "0.1.11"
+pretty_env_logger = "0.3"
+structopt = "0.3"