Add convenience constructor methods to taproot classes

[jnewbery]

• [ ] add Node(left, right) constructor (https://github.com/bitcoinops/taproot-workshop/pull/19#discussion_r326712417)
• [ ] add TapLeaf(descriptor_string) constructor (https://github.com/bitcoinops/taproot-workshop/pull/19#discussion_r326712118)

Suggested by @bitschmidty

[jnewbery]

add Node(left, right) constructor

Implemented in #137