Fix parsing of Double literal trailing whitespace - #1647

The fix from #1647 broke GHCJS support for Dhall.  Specifically, any Dhall
code containing numeric literals would fail to parse when using GHCJS.

The consequence of this is that the final example from the online demo
currently does not work because it uses at least one number.

The root cause was this issue:

ghcjs/ghcjs-base#62

The change to use `Text.Megaparsec.Char.Lexer.float` triggered the above
problem, because `float` depended transitively on `Data.Char.toTitle` which
was not working for the version of GHCJS that we currently build against.

The fix is to reimplement the logic for parsing double literals by
following the standard grammar instead of reusing an existing utility.
However, I cribbed quite a bit from the existing logic for
`Text.Parser.Char.double` along the way.

* Fix numeric parsing for GHCJS

The fix from #1647 broke GHCJS support for Dhall.  Specifically, any Dhall
code containing numeric literals would fail to parse when using GHCJS.

The consequence of this is that the final example from the online demo
currently does not work because it uses at least one number.

The root cause was this issue:

ghcjs/ghcjs-base#62

The change to use `Text.Megaparsec.Char.Lexer.float` triggered the above
problem, because `float` depended transitively on `Data.Char.toTitle` which
was not working for the version of GHCJS that we currently build against.

The fix is to reimplement the logic for parsing double literals by
following the standard grammar instead of reusing an existing utility.
However, I cribbed quite a bit from the existing logic for
`Text.Parser.Char.double` along the way.

* Add comment explaining the hand-written parsing code

... as suggested by @sjakobi

* Work around GHC parsing bug

For some reason `-- * ${URL}` is not allowed in a comment?