Add to terminal new Operation for showing free variables of an expression

[jmansilla]

Example:

λh> zero = lambda x.x
new expression parsed: zero = (λx.x)
λh> zero -> show_free_variables
OUT: []
λh> two = lambda x.y z
new expression parsed: two = (λx.(y z))
λh> two -> show_free_variables
OUT: [y, z]