Lint sensitivity lists

[Schottkyc137]

Closes #359

Add a linter for sensitivity lists.

For the most part, the linter is designed to be conservative concerning error codes and only emit warnings when absolutely certain. This means that a couple of potential problems will not be raised.

New error codes

• missing_in_sensitivity_list (default: warning): warns when there is a signal missing in the sensitivity list of a process that is read in a combinational process.
• superfluous_in_sensitivity_list (default: warning): warns when there is a signal in the sensitivity list of a process that is not read inside the process

Caveats

The linting only applies to combinational processes. To differentiate whether a process is combinational or sequential, a heuristic is applied.
Every process that contains any numbers of if statements with conditions that describe clocked elements are considered sequential. Clocked elements are either rising_edge(clk) or clk'event. This means that especially mixed processes like

process(clk) is
begin
     if rising_edge(clk) then
         -- sequential logic
    end if;
    a <= b;
end process;

won't be linted.