Rework expansion logic

[yobert]

The previous logic would have some bugs if you had a tree like this:

key = value
a   = ${b} ${key} ${c}
b   = ${key}
c   = ${key}

This reworks the expand() logic to use a stack which is copied at each
level of recursion. This also fixes bugs when you have more than two
expansions, like this:

a = b
c = ${a}${a}${a}

[magiconair]

Released v1.7.6.