Add support for merge keys

[Timothy-Gonzalez]

YAML merge keys, as defined here should be supported.

While not part of the standard specification, it is still recommended to support as:

The use of these tags is not mandatory. However these tags represent types that are useful across a wide range of applications and it is strongly recommended they be used whenever appropriate to promote interoperability.
Source

This will technically be a breaking change.

Example Behavior

a: &a #Anchor
  b: 1
c: *a #Normal alias
d:
  <<: *a #Merge alias
  e: 2

Would result in:

a:
  b: 1
c:
  b: 1
d:
  b: 1
  e: 2

Tasks

• Find crate which supports yaml merge keys and integratable support for serde_yaml
• Implement merge keys
• Create merge_key test

[Timothy-Gonzalez]

Possible canidate: yaml_merge_keys. It even has built-in support for serde_yaml through the serde_yaml feature.

• crates.io
• docs.rs