Improve one-liner support

[jprochazk]

• Semicolons #34

Allowed as statement terminators, in which case the next statement does not need to be on another line:

v := 0; print v # prints `0`

If the statement is on another line, normal indentation rules still apply:

v := 0;
  print v
# ^ error: invalid indentation

• Pipeline operator

If the rhs is a single identifier, it is treated as a function with a single parameter.
If the rhs is any other expression, it must have at least one argument which is the identifier _ (maybe consider making this a keyword) - it is used as a placeholder, into which the expression on the lhs will be placed. The placeholder may appear multiple times in the rhs, but it is only evaluated once.

a() |> b |> c(_, option=true)

# desugaring:
_0 := a()
_1 := b(_0)
_2 := c(_0, option=true)


print (100 |> _ * _) # prints `10000`

# desugaring:
_0 := 100
_1 := _0 * _0
print _1