fix switch statements

[watermarkhu]

Fixes #45 Fixes #47 Fixes #52 Fixes #64

Switch statements are syntactically the same as if/elseif/else.

switch	if
switch keyword	if keyword
switch declaration	if declaration
-	if statements
case keyword	elseif keyword
case declaration	elseif declaration
case statements	elseif statements
otherwise keyword	else keyword
otherwise statements	else statements
end keyword	end keyword

This PR copies the implementation from #82 and replaces the tokens with the ones for switch.

[dklilley]

Thanks for making these changes!