`where` with final result at end

[evhub]

Possible syntaxes:

out = where:
    x1 = a + b + c
    x2 = x + y + z
    x1 + x2

out = do:
    x1 = a + b + c
    x2 = x + y + z
    x1 + x2

Which should compile to the equivalent of:

out = x1 + x2 where:
    x1 = a + b + c
    x2 = x + y + z