Detailed error messages

[justjake]

This PR builds on the changes in #6, so it'd be best to merge that before reviewing, or start reviewing after the duplicated commits.

Major changes:

• Add .toString() method to all Types which returns a typescript-like representation of the type instance. This allows us to print types in error messages.
• A much richer Err type that tracks path, error causes, and error types.
  • Err messages can be lazy () => string instead of string, avoiding expensive string building unless an exception is being raised, which makes Type#sliceResult even expensive.
• A custom Error class thrown for assertions, StructuralError, which collects the root causes of a type error, enumerating the keys that did not match an expected type.

Example of exception output:

StructuralError: given value
  { owner: { username: 'the duke', age: 42, email: 'duke.nuke@example.com' },
    contents: 
     [ { text: 'Those who do not study history are doomed to repeat it',
         author: 'Albert Einstien',
         date: 1970-01-01T00:00:02.001Z },
       { uri: 'example.com',
         alt: 'Mr. Doctor Albert Einstien wearing a tweed jacket' } ] }
did not match expected type
  { owner: { username: string,
      age?: number,
      email: string & validate(has @ symbol, (val) => !!val.match(/@/)) },
    contents: Array<{ text: string, author: string, date?: Date } |
    { uri: string & validate(starts with http, (val) => !!val.match(/^http/)),
      alt: string }> }
because: at .contents[1]: given value
    { uri: 'example.com',
      alt: 'Mr. Doctor Albert Einstien wearing a tweed jacket' }
  did not match expected type
    { text: string, author: string, date?: Date } |
    { uri: string & validate(starts with http, (val) => !!val.match(/^http/)),
      alt: string }
  because: matched none of 2 types:
    | { text: string, author: string, date?: Date }
      failed multiple checks:
      - at .text: given value `undefined` did not match expected type `string`:
        key missing
      - at .author: given value `undefined` did not match expected type `string`:
        key missing
    | { uri: string & validate(starts with http, (val) => !!val.match(/^http/)),
        alt: string }
      at .uri: given value `'example.com'` did not match expected type
        validate(starts with http, (val) => !!val.match(/^http/))
      because: validation returned false

That exception has these root causes:

se.causes.forEach(c => { console.log(c.toString()); })
  console.log test/errors.ts:141
    at .contents[1].text: given value `undefined` did not match expected type `string`:
      key missing

  console.log test/errors.ts:141
    at .contents[1].author: given value `undefined` did not match expected type `string`:
      key missing

  console.log test/errors.ts:141
    at .contents[1].uri: given value `'example.com'` did not match expected type
      validate(starts with http, (val) => !!val.match(/^http/))
    because: validation returned false

[justjake]

@reissbaker bump