The purpose of this commit is the start of a tool to generate Ion
formatted ASTs from Pest grammars, particularly that of PartiQL.
We could then easily process the grammar outside of Rust in scripts that
could be used to generate things like TextMate grammars for IDE support
or LaTeX fragments for the specification document.

Adds the `pestion` crate to provide a simple trait `PestToElement` and
implementations over the Pest AST and `&str` syntax definitions to
create Ion `Element` serialized forms.

Resolves partiql#35.

As per feedback from @zslayton.

Renames `PestToElement` to `TryPestToElement` which captures the
fallible conversion (e.g. from `str` slices) and makes `PestToElement`
be an infallible version of the API.  This is a more correct factoring,
but also needed to avoid the intermediate copy for `Vec<AstNode>`
conversion.

Changes `AstRule` structure generation to use an array as it has a fixed
number of fields.

Adds `smallvec` to make it easier to model dynamically sized yet stack
allocated fields for sub-structures for `Expr` conversion.

Adjusts the doc test to be more illustrative.

Updates surrounding meta-data and README.

Also renames the following types:
* `PestionResult` => `PestToIonResult`
* `PestionError` => `PestToIonError`