eval is missing types, causing dialyzer to complain

[ArthurClemens]

Current types:

@spec eval(expr :: String.t | charlist) :: {:ok, result::boolean} | {:error, error::map}
@spec eval(expr :: String.t | charlist, props :: map) :: {:ok, result::boolean} | {:error, error::map}

With missing types added:

@type expreso_result ::
        {:ok, boolean()}
        | {:error, map()}
        | {:error, String.t()}
        | {:error, {term(), :expreso_parser, term()}}
        | {:error, {term(), :expreso_lexer, term()}, term()}
@spec eval(expr :: String.t() | charlist) :: expreso_result()
@spec eval(expr :: String.t() | charlist, props :: map) :: expreso_result()