Validation changes

[thejohnfreeman]

• Rename "lens" to "scratch". Other suggestions include "tunnel", "gate", "prism", "source", etc. but below discussion will use "scratch".

• No error on target variable. Binders can distinguish variables and scratches. A binder (e.g. the number binder) creates a scratch as convenience only when needed. The programmer can bind to an error only if they explicitly created the scratch for it.

• No declarative validation, e.g., toModel, toView, validate in a data-bind attribute. The programmer must create a scratch to add custom validation. We will reserve the option to allow, in the future, declarative addition of custom validation.

• Validating binders prepend their validation when binding to an existing scratch. It is an (undiagnosed) error to duplicate a binder's validation when creating a scratch for the binder. For example,

  <input data-bind="number: y">

  var y = hd.scratch(x).number().min(0); // wrong
  var y = hd.scratch(x).min(0); // right

• Validator conveniences should be extensible. Programmers should be able to extend new scratch proxies with custom validators.

• .outgoing should be variadic. When called with multiple validators, they should all be appended.