Composable contracts

This feature will allow contracts to be composed of other contracts, optionally mounted at input paths.

## Example

```ruby
# this contract defines its own params and rules, but also uses the
# schema/rules of CustomerContract, and AddressContract on the input at :address
# via the #contract and #path methods
class OrderContract < ApplicationContract
params do
required(:accept_terms).value(:bool)
end

rule(:accept_terms).validate(:acceptance)

contract CustomerContract

path :address do
contract AddressContract
end
end
```

## Implementation

- Initial on branch [composable-contracts](https://github.com/dry-rb/dry-validation/tree/composable-contracts)
- [x] Create a **:composable** extension
- [x] implement new #contract feature in the :composable extension
- [x] Add #path block DSL
- [ ] Rewrite ResultSet as extension to Result
- [ ] Rewrite the [contract/call_spec](https://github.com/dry-rb/dry-validation/blob/master/spec/integration/contract/call_spec.rb) integration test using the desired new syntax
- [ ] Add an example
- [ ] Document the changes in docsite

- Future work
- Decide on exact semantics for merging result values/errors if its unclear
- Ensure it works wrt: concurrency
- Figure out if we should integrate with other extensions in the short term
- Decide on a syntax so contracts can be applied to nested **arrays** of input
- Decide on a syntax so contracts can be conditionally be applied depending on the input
- If consensus that the API is good, remove the :composable extension and rewrite contract and result to include the new functionality by default

## Resources

- Discussion on the [dry-rb forum](https://discourse.dry-rb.org/t/thoughts-on-dry-validation-wrt-composing-validations/904/3)
- [Code spike gist](https://gist.github.com/ianwhite/a32fcd439020ea07ed1fe3243152274f)
- [Pasteable in irb example](https://gist.github.com/ianwhite/918c7ba3487e05b2272a42c154e6b6fb)