Make basic one-line statements work
Make basic one-line statements work

Currently, statements can only be one line in length. They will be
executed in the current `_context`, which is similar to what happens
in a normal interactive Python interpreter too.

A lot is still left to do. For example, the evaluation function should
detect if more input lines will come (e.g. when an input line starts with
`while`), and construct a multiline statement out of whatever comes
until two successive newlines are seen. This would at least let us do
things like print close-to-live accelerometer values on @SensibilityTestbed.

aaaaalbert/repy-doodles