modular schema

would be great to have an `$include` syntax, in the form:

`$include as x` - to import the Start schema in that file as an an available schema under name x
`$include x in as y` - to import the named schema in that file (x) as a different name (y)
`$include x in ` - to import the named schema in that file under its original name

this would allow for a good deal of code reuse, especially with the 'parameterized schema' feature request