[feature] define bounds in contract

[RiedleroD]

for example:

final random = contract('Random')
  .impl('number', returnType: PrimitiveType.DOUBLE, returnBounds: [0.0, 1.0])
    .param('seed', PrimitiveType.INT, bounds: [0, 2**32])
  .end()
  .hook('onReady')
    .param('user', PrimitiveType.STRING, minlength=2, maxlength=13)
  .end()
  .build();

the exact impl is not that important, but having an easy way to limit the values that go through the contract would be very neat indeed.