If a plugin calls `PluginContext.parse` method to parse the AST of the module
rollup ignored the `PURE` annotations. This commit fixes the issue by moving
the annotation functionality to a central location - the `contextParse` method.

Resolves rollup#3979

Use the `onComment` feautre. This expands the coverage.

This change eliminates needless mapping between the types.

When parsing the JS code we add attributes to acorn.Node for later use:
* annotations: acorn.Commnet - the comment that defined the annotation
* annotatedPure: boolean - declares this node as pure call/new expression

This commit puts all these attributes under one name: `_rollupAnnotations` with
a strong type.

Thus we clarify that these attributes are for internal use only and decrease
the change of ever coliding with internal/future attributes of `acorn.Node`.