Implement Default for Builder
Implement Default for Builder

Clippy emits:

  warning: you should consider adding a `Default` implementation for
  `Builder`

We have a `new` constructor that takes zero args; for interoperability
with other Rust standard code add a `Default` implementation for the
`Builder` type by way of the `new` method.

apoelstra/rust-jsonrpc,apoelstra/rust-jsonrpc