Fix some warnings

[elikoga]

Removes redundant imports issuing warning: [-Wunused-imports] on compile time

Adds type signatures to literals issuing error: [-Wtype-defaults, -Werror=type-defaults] on compile time when compiled with stack build --pedantic

[aquarial]

The import Data.Monoid ((<>)) let's the project build on an older version of base that doesn't import it by default.

I get home Sunday or so, and I'll take a look to see if that base is so out of date it's just worth dropping, or we should use compiler pragma guards or something.

I agree the pile of warnings isn't nice.