implement contexts for autoload import/export
Merge branch 'pr176' of https://github.com/sn6uv/mathics into wip/context
Remove Definitions.autoload_stage

This was problematic because it made every definition in an autoloaded
file a builtin. Builtin definitions can't be removed, implying that a
Block[] can't be executed inside an autoloaded file.
Use Get to read autoloaded files

Get reads the file line-by-line, which allows $Context changes in
autoloaded files to take effect.

To allow the the JSON importer to be loaded through Get, make
wait_for_line ignore brackets inside strings. Otherwise Get reports a
false parse error.
Update Contexts and Names tests

Remove import/export related symbols from Names, add new contexts to
Contexts, and make sure a Global` definition exists when testing
Contexts.
Make autoloaded definitions builtins again