Support expressions as the body of a validation

For example:

```elixir
validate expr(condition), error_expr: expr(error(...))
```

Start by locating the validation DSL and the expression-handling entry points; the issue names no files or tests. Use the example with expr(condition) and error_expr: expr(error(...)) as the starting case, and consider the work complete when that validation form is accepted and behaves correctly.

Written by the indexing model from the issue text.