This rule requires adding a new set of rules, tslint-etc.
no-unused-declaration used to be available in tslint:recommended but was
deprecated when --noUnusedVariables was added to typescript. The problem
with using TypeScript's --noUnusedVariables is that it turns unused
declarations into an error and prevents compilation, which isn't fun
when you're just prototyping things.
This commit makes the compiler pass use different classes in order to
represent the metadata. This enables adding per-class toString/convert
functions. This enables easy type checking and conversion in the `:set`
excmd.
