Require whitespace before with

[travisbrown]

The grammar currently requires whitespace before with:

with-expression =
    import-expression 1*(whsp1 with whsp1 with-clause)

I'm following this in Dhall for Java, but noticed that the Haskell implementation doesn't match the standard:

$ dhall <<< "{=}with foo = 0with bar = 1"
{ bar = 1, foo = 0 }

This seems to me like a bug on the dhall-haskell side, not in the grammar, but I'm happy to open an issue over at dhall-lang if anyone disagrees.

[Gabriella439]

Fixed by #2213