Dictionaries

[simonask]

Might be cool to have dictionaries (mappings where the key is always a string). Dictionaries should use IndexMap to preserve insertion order.

• Dictionary lookup syntax dict[K] in expressions.
• Dictionary lookup syntax in string interpolations: {dict[K]}
• Chaining ops: keys, values, merge, flatten, first, last, len, head, tail
• Expressions: read-json, read-yaml, read-toml
• Run ops: write-json, write-yaml (TOML is harder, but possible)

This would be useful to support native JSON, TOML, or YAML lookup operations.