Initial methods for Port.Input

[samdphillips]

A few initial methods for reading from Port.Input. Documentation adjusted from the associated Racket functions. The functions all have a required Port.Input argument, unlike the Racket equivalents which use a default parameter.

[mflatt]

JFYI: This doesn't build for me due to some problem in compiler/demod. But using syntax-parse instead of define-syntax-parse-rule avoids the problem, and that change needed anyway to minimize dependencies. I'm preparing the change and some other proposed changes to the PR.

[mflatt]

I pushed some extra commits with the following goals:

• Stick to just syntax/parse/pre as the syntax-parse dependency to avoid pulling libraries like the contract system.
• Put define-simple-symbol-enum in its own module, since a goal in this code is to split things into modules as much as can make sense.
• Make sure read_line reports an error in terms of Port.Input.ReadLineMode
• Adjust the docs to mostly use raw symbols. The main goal here is to avoid something verbose like Port.Input.ReadLineMode = Port.Input.ReadLineMode.any in favor of Port.Input.ReadLineMode = #'any. I'm less sure that symbols should be used in the enumeration instead of the dotted names.
• Add tests to check that the new methods really can be called.

I also ended up filling in the Char class.

[mflatt]

Ok if I squash to merge? Or would you like to keep the individual commits or some different organization?

[samdphillips]

Squash is fine