Use structopt instead of docopt.

This commit refactors the Wasmtime CLI tools to use `structopt` instead of
`docopt`.

The `wasmtime` tool now has the following subcommands:

* `config new` - creates a new Wasmtime configuration file.
* `run` - runs a WebAssembly module.
* `wasm2obj` - translates a Wasm module to native object file.
* `wast` - runs a test script file.

If no subcommand is specified, the `run` subcommand is used. Thus,
`wasmtime foo.wasm` should continue to function as expected.

The `wasm2obj` and `wast` tools still exist, but delegate to the same
implementation as the `wasmtime` subcommands.  The standalone `wasm2obj` and
`wast` tools may be removed in the future in favor of simply using `wasmtime`.

Included in this commit is a breaking change to the default Wasmtime
configuration file: it has been renamed from `wasmtime-cache-config.toml` to
simply `config.toml`.  The new name is less specific which will allow for
additional (non-cache-related) settings in the future.

There are some breaking changes to improve command line UX:

* The `--cache-config` option has been renamed to `--config`.
* The `--create-config-file` option has moved to the `config new` subcommand.
As a result, the `wasm2obj` and `wast` tools cannot be used to create a new
config file.
* The short form of the `--optimize` option has changed from
`-o` to `-O` for consistency.
* The `wasm2obj` command takes the output object file as a
required positional argument rather than the former required output *option*
(e.g. `wasmtime wasm2obj foo.wasm foo.obj`).

Cargo.lock

@@ -316,7 +316,7 @@ dependencies = [
  "ansi_term",
  "atty",
  "bitflags",
- "strsim 0.8.0",
+ "strsim",
  "textwrap",
  "unicode-width",
  "vec_map",
@@ -550,18 +550,6 @@ dependencies = [
  "winapi",
 ]
 
-[[package]]
-name = "docopt"
-version = "1.1.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7f525a586d310c87df72ebcd98009e57f1cc030c8c268305287a476beb653969"
-dependencies = [
- "lazy_static",
- "regex",
- "serde",
- "strsim 0.9.2",
-]
-
 [[package]]
 name = "dynasm"
 version = "0.5.2"
@@ -1185,6 +1173,17 @@ dependencies = [
  "log",
 ]
 
+[[package]]
+name = "proc-macro-error"
+version = "0.2.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "aeccfe4d5d8ea175d5f0e4a2ad0637e0f4121d63bd99d356fb1f39ab2e7c6097"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "proc-macro-hack"
 version = "0.5.11"
@@ -1622,10 +1621,27 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8ea5119cdb4c55b55d432abb513a0429384878c15dde60cc77b1c99de1a95a6a"
 
 [[package]]
-name = "strsim"
-version = "0.9.2"
+name = "structopt"
+version = "0.3.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "032c03039aae92b350aad2e3779c352e104d919cb192ba2fabbd7b831ce4f0f6"
+checksum = "30b3a3e93f5ad553c38b3301c8a0a0cec829a36783f6a0c467fc4bf553a5f5bf"
+dependencies = [
+ "clap",
+ "structopt-derive",
+]
+
+[[package]]
+name = "structopt-derive"
+version = "0.3.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ea692d40005b3ceba90a9fe7a78fa8d4b82b0ce627eebbffc329aab850f3410e"
+dependencies = [
+ "heck",
+ "proc-macro-error",
+ "proc-macro2",
+ "quote",
+ "syn",
+]
 
 [[package]]
 name = "syn"
@@ -1950,12 +1966,10 @@ name = "wasmtime"
 version = "0.7.0"
 dependencies = [
  "anyhow",
- "docopt",
  "file-per-thread-logger",
  "pretty_env_logger",
  "rayon",
  "region",
- "serde",
  "target-lexicon",
  "thiserror",
  "wasi-common",
@@ -1973,14 +1987,13 @@ name = "wasmtime-cli"
 version = "0.7.0"
 dependencies = [
  "anyhow",
- "docopt",
  "faerie",
  "file-per-thread-logger",
  "libc",
  "more-asserts",
  "pretty_env_logger",
  "rayon",
- "serde",
+ "structopt",
  "target-lexicon",
  "test-programs",
  "wasi-common",