Proposal: define types at top file level

Allow users to define types at top level of files. Parameters can reference these in the `type` field.

Example:

```
- id: FooType
type: string
inputBinding:
prefix: "-o"

- id: BarTool
type: CommandLineTool
inputs:
- id: output
type: "#FooType"
```

This obsoletes SchemaDefRequirement.

Start by reading the CWL specification and its SchemaDefRequirement definition to understand how top-level types and parameter type references are currently represented. Use the YAML example in the issue as the target behavior, and confirm that the new form makes SchemaDefRequirement obsolete without leaving type resolution ambiguous.

Written by the indexing model from the issue text.