Be more explicit about range syntax, and throw syntax errors as appropriate

[mccalluc]

Superficially, our range syntax is similar to python's. A number of bugs already filed. Additionally:

• Negative step value?
• Leave off the first index to get everything from the start?
• Or leave off the last index to get everything till the end?

I think it's fine not to support this full syntax, but we should be more clear about what the limits are:

• The exact syntax should be documented, and
• if that syntax is violated there should be an error, instead of a silent failure.