add tests and check notes constraints

this is for #5

• test constraint on needing either a file or a directory ref
• test unique parent and name constraint
• test unique dir constraint
• try adding a constraint that a dir ref must actually point to a dir entry
• test unique md file constraint
• try adding a constraint that an md file actually points to a(n md) file
• add function for listing the children of a given note, and add it to some tests.

the additional constraints would require multiple, complicated queries on inserts and possibly updates. not really worth the effort or the performance hit.