Extend the Expression type with a constructor for unescaped literals

[cgohla]

We add UnescapedLiteral :: ByteString -> Expression t as an escape hatch.

As an example, this is needed to produce this piece of Bash output:

bar="([name]=Bart [age]=12)"
declare -A foo="$bar"

The reason we need the escape hatch here is that the Dict constructor of the Assignment type imposes too much structure on the RHS of the assignment.