Feature request: partial loading using path-like specifier

[pdgendt]

For use-cases where only part of a document is needed, it would be nice to be able to load and return early.

For an example document:

a:
  b:
    - x
    - y
    - z
  c:
    d: 42

We could partially load, for example:

print(load_part(yaml_text, ".a.c.d"))   # -> 42
print(load_part(yaml_text, ".a.b[1]"))  # -> "y"
print(load_part(yaml_text, ".a.c"))     # -> {'d': 42}
print(load_part(yaml_text, ".a.b"))     # -> ['x', 'y', 'z']
print(load_part(yaml_text, ".nonexistent"))  # -> None

Advantages (depending on usage of anchors):

• Only materialize values that lie along the requested path.
• Stop as soon as we finished building the value at that path.