Add top level exports and top level errors, fix flow

Contains #96, #97

• BREAKING CHANGE: throw TypeError instead of Error if wrong value is passed to an unmutable function
• fix: removed broken flow files. Will add these back in manually as I work on the docs (#53), rather than via a script.
• add: improve error message
• add: throw errors from top level unmutable file #94
• add: publish function files from top level, so import map from 'unmutable/map' will work #93
• refactor: add jest lint
• refactor: rename prep.js to unmutable.js so origin of errors is clearer