Interesting deserialize_with example

[dtolnay]

@locallycompact had this use case:

assets/test-stratum.morph:

- repo: http://example.com/repo.git
  ref: aaaaaa
  bit: assets/test-bit.bit

assets/test-bit.bit:

configureCommands:
- "foodedoo"
buildCommands:
- "ladida"
installCommands:
- "dooopdedoop"

One file contains the path to another file. We can use deserialize_with to inline the contents of the second file into a struct representing the first.