Fix broken if_else_stmt production

[zl90]

Currently broken with the following input:

if x {
    g = 5
}
else { // not sure the else statement should be on a newline anyway. Check the language spec
    g = 2
}

Also needs to be able to have no curly braces with a single statement, like so:

if (a < b) b = b - a
else a = a - b