New rule: `DisallowedDeclarationName`

[a-frantz]

Credit to @adthrasher for the idea proposed in a comment on #133

We have DisallowedInputName and DisallowedOutputName. Those should remain unchanged, but there should be a new rule that checks all Decls (IO and private) for "type prefixes/suffixes". e.g. File gtfffile would be bad and so would Int my_int