Refactor: AST

[LesterEvSe]

ast.rs had grown to approximately 4 800 lines. this PR splits it into smaller files. There is no change in behavior or public API.

Structure

src/ast/
├── mod.rs           AST node types (Program, Expression, Call, ...) and their accessors
├── jet_hinter.rs    JetHinter trait, ElementsJetHinter, CoreJetHinter
├── recovery.rs      Failure and the rule for which errors are suppressed (multi-error)
├── scope/
│   ├── mod.rs       Scope: variables, modules, witnesses, parameters, diagnostics
│   └── imports.rs   resolution of `use` declarations
├── analyze/
│   ├── mod.rs       the AbstractSyntaxTree trait
│   ├── program.rs   Program, items, functions
│   ├── statement.rs Statement, Assignment
│   ├── expression.rs Expression, SingleExpression
│   ├── call.rs      Call, CallName
│   └── matching.rs  Match, EnumMatch, EnumConstruction
└── tests/           tests that check source in -> errors out

Checklist

• If this PR adds or changes a language feature, it is used in
  examples/kitchen_sink.simf, and test-data/kitchen_sink.json is
  regenerated.

[apoelstra]

ACK 63b23e5; successfully ran local tests; this is great!