Skulpt notes

We need to get more context into errors that are raised by the Compiler. The objects passed into Compiler's methods seem to be AST nodes, with properties that we can use:
- lineno - the line number (1-based) of the token
- col_offset - the column number (0-based) of the token
Each AST node has its own properties as well; for example, for-loops have a body property with the node representing the loop body, print statements (and other functions?) have a values property which is an array of argument nodes, binops have left and right nodes, etc.
Need to do the same for runtime errors (like division by zero); how do we figure out the context in those cases?
The Skulpt Exception classes have the following constructor signature:

Exception(message, filename, lineNumber, columnNumber, context)

The first four are self-explanatory; context is the text of the line where the error occurred. (The built-in str method uses this to print the error message with a caret pointing at the location of the error.)

Here's the catch: Once the values are stored in the exception, they're converted to Skulpt tuples and string objects; in other words, objects with a v property that contains the actual value.

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