Update to clap 3.* (#4082)

* Update to clap 3.0

This commit migrates all CLI commands internally used in this project
from structopt/clap2 to clap 3. The intent here is to ensure that we're
using maintained versions of the dependencies as structopt and clap 2
are less maintained nowadays. Most transitions were pretty
straightforward and mostly dealing with structopt/clap3 differences.

* Fix a number of `cargo deny` errors

This commit fixes a few errors around duplicate dependencies which
arose from the prior update to clap3. This also uses a new feature in
`deny.toml`, `skip-tree`, which allows having a bit more targeted
ignores for skips of duplicate version checks. This showed a few more
locations in Wasmtime itself where we could update some dependencies.

crates/wiggle/generate/Cargo.toml

@@ -17,7 +17,7 @@ include = ["src/**/*", "README.md", "LICENSE"]
 witx = { version = "0.9.1", path = "../../wasi-common/WASI/tools/witx" }
 quote = "1.0"
 proc-macro2 = "1.0"
-heck = "0.3"
+heck = "0.4"
 anyhow = "1"
 syn = { version = "1.0", features = ["full"] }
 shellexpand = "2.0"