Parse Strings surrounded by `"`

Parse Strings surrounded by `"`

We need to use a `do-block` because we would like to keep the return
value of a parse in the middle. So we parse a single `char '"'`, parse
`many (noneOf "\"")` and assign the return value to `x` and parse
another `"`.

We apply the `String` constructor to the value of `x` to turn it into a
`LispVal` with `String x`.

Finally we inject the `LispVal` into the `Parser` monad. We need to do
this because each line of a `do-block` needs to be of the same type but
the result of `String x` is a `LispVal`, not a `Parser`. `return` wraps
the `LispVal` up in a `Parser` action that consumes no input.

Consider `a $ b c` as syntactic sugar for `a (b c)`.