This commit is a step to split the lnwallet package. It puts the Input
interface and implementations in a separate package along with all their
dependencies from lnwallet.
This commit adds a tutorial on fuzzing with the go-fuzz library
into the docs folder. It includes an introduction to fuzzing,
setup and installation steps to run go-fuzz with lnd, tips to
generate a valid corpus for use with go-fuzz, and finally it
includes a small explanation of the test harness that was used
to find bugs in lnd.