Make variable declarations immutable by default

[liquidev]

Currently it's easy to overwrite existing variables by accident, due to all assignments mutating the outermost variable. Instead of this, I think that by default creating a new variable each time would make more sense, with an optional mut keyword that makes a variable mutable, and prevents further declarations from shadowing existing ones:

x = 1
do
  x = 2
  assert(x == 2)
end
assert(x == 1)

mut i = 1
while i <= 10 do
  i = i + 1
end