Prohibit certain strings from being used as type names or schema IDs

[popematt]

Ion Schema 1.0 does not place any restrictions on the strings used for schema IDs or type names.

Most programming languages do have restrictions on the strings that you can use for identifiers, so such restrictions are not unreasonable in general. The purpose of such restrictions is to avoid ambiguous cases when tokenizing or otherwise processing code in that language. A secondary purpose is to stop people from doing things that are generally a bad idea. The Ion library(s) already take care of tokenizing and loading the schemas documents into the Ion DOM, but it would make the Ion Schema implementation easier (particularly the violation messaging) if certain classes of strings were prohibited.

We may want to prohibit the following types of strings from being used as type names or schema IDs:

• The empty string
• Strings with leading or trailing whitespace
• Strings that contain newlines
• Strings that contain non-printable characters

[popematt]

Closing in favor of #60, since this would be better off as a linter rule.