Add witx crate, to parse and validate witx

[pchickey]

As a follow on to #74, I'm submitting the witx crate, adapted from the Lucet project (https://github.com/fastly/lucet).

witx parses and validates a witx file into the representation given in tools/witx/src/ast.rs. The validation rules are written in src/validation.rs. The crate produces a binary, witx, which validates a witx file, or reports an error if it fails to validate.

witx uses stable rust, and has only two crate dependencies - failure, a very-nearly-standard error reporting library, and clap, a popular argument parsing library. (clap and the executable could be separated out into a separate crate if necessary to drop that dependency).

This PR also fixes one minor syntax error in the existing witx files. It is already useful!

After this is merged, I will make a PR to add Azure Pipelines integration to ensure that this code builds without errors, and validates all checked-in witx files, as a condition to merge each PR.

[sbc100]

I guess its time I learnt this newfangled language.

[sbc100]

Should we put a copyright header at the top of each source:

/*                                                                               
 * Copyright 2019 WebAssembly Community Group participants                       
 *                                                                               
 * Licensed under the Apache License, Version 2.0 (the "License");               
 * you may not use this file except in compliance with the License.              
 * You may obtain a copy of the License at                                       
 *                                                                               
 *     http://www.apache.org/licenses/LICENSE-2.0                                
 *                                                                               
 * Unless required by applicable law or agreed to in writing, software           
 * distributed under the License is distributed on an "AS IS" BASIS,             
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.      
 * See the License for the specific language governing permissions and           
 * limitations under the License.                                                
 */                                                                              

[sbc100]

And maybe a LICENCE at the top level? (or maybe the top level of the entire repo?)

[sunfishcode]

The spec interpreter doesn't have copyright notices in the source files, and I think it's a reasonable example to follow.

[pchickey]

Added a LICENSE file with that exact text, fixed the license field in cargo toml to leave out the llvm-exception part

[pchickey]

Per @sunfishcode's suggestion, I renamed the crate to witx, since it will be essentially the standard witx implementation for Rust.

[pchickey]

I also published this version to crates.io to reserve the package name, and added Dan as an owner.

[sunfishcode]

Looks good! Since we discussed this in the last meeting, let's go ahead and merge, and we can continue to iterate with PRs and issues from here.