Merge maps and lists in YAML blocks instead of left-biased union

Always ignoring extant keys (pure left-biased union) can be a bit counterintuitive. It may be better to merge the contents of mappings and lists, then use left-biased union for conflicting scalar values, so that
```
---
one:
    first: 1
---
---
one:
    first: 2
    second: 3
---
```
should simplify to
```
---
one:
    first: 1
    second: 3
---
```