Consistently use quotes in parsing errors

Use quotes around identifier and operators, as well as around
parsed expressions to make understanding messages easier.

This is a change I felt the need for after getting a message like
> ERROR: unrecognized named argument types

which is much clearer as
> ERROR: unrecognized named argument "types"

This can be even more useful when a weird user-typed expression that could not be parsed is printed.

I've reused the convention to use double quotes, which I found in one string. Another string used single quotes around ``:``. If you prefer single quotes in some cases, and double quotes in other, I can adapt the commit. But I find such a convention very hard to respect in a large code base, let alone package authors.

I'm not familial at all with Scheme, so I hope I've not broken anything. Most cases are simple, but you should probably have a look at the lines I changed that contain ``#\.`` and ``':`` (i.e. line 206 and line 534).