Make JSON builders immutable (breaking)

Breaking changes

• JsonBuildObject is now immutable and again a var instead of a function (because it is not mutated anymore)
  • Make sure to update code to always use the return value of any mutating call to the builder

Other changes

• Added more functions to the json package: JsonBuildArray and ExpIfNotNull
• Added a complete example with Squirrel / pgx showing many features

Add JSON serialization for writes

• Complex types can now easily be embedded as JSON (e.g. jsonb column in PostgreSQL) by adding the json option to the write_col struct tag

Breaking: Generate function for default select JSON

Construct will now generate a function to get a default JSON object builder for selects since the builder might be mutated.

Generate schema consts and JSON scan func

This release adds two new features to the generator:

• Generate constants for all read columns
• Generate a function to scan a JSON row result into the target type

See README.md for examples.

First version (experimental)

v0.1.0

Add readme, fix format