Use semicolons as delimiters in WIT text format

[alexcrichton]

This commit commit is an implementation of #142 where semicolons are now required as delimiters between items in the WIT text format. All items in the WIT format are now delimited with either curly braces ({}) or semicolons except for the package statement where it subjectively felt a bit weird to require a semicolon. I've updated the various examples in WIT.md as an example of the new syntax.

My plan on implementing this would be along the lines of:

• Implement the semicolon syntax in wit-parser
• Add a parser mode which requires semicolons. This means that the same wit-parser crate can either or either not require semicolons.
• Update all tests in the wasm-tools repository to require semicolons.
• Publish wit-parser and wasm-tools, integrating the semicolon-supporting-mode into all existing tools.
• Wait for Wasmtime to get published with this support. At this point everything in the ecosystem should have a point where semicolons are optionally supported.
• Remove the parser mode which doesn't require semicolons, meaning semicolons are now required.
• Push this update through the tooling, fixing any issues that arise.

The hope is to create a period of time where both syntax forms are accepted. This provides a transitionary means from one syntax to the other while proposals are updated. This transitionary period is finite in length, however.

Closes #142

[alexcrichton]

I've got a PR for an implementation of this when it's ready at https://github.com/bytecodealliance/wasm-tools/pull/1212 which can also serve as a means to evaluate the aesthetics of the change via reading all the changes to tests. (aesthetics improved IMO)

[lukewagner]

Looks like no disagreement, so merging.