Rethink argument system

TypeString - Relates to #38

ForeignUrl - relates to #23

# Current notes

## More thoughts: piggy back on profile code!

Perhaps the way to think of this is "automatic conversion profiles". If no conversion profile(s) are specified, we get a default one that is the result of back-propagating all relevant arguments based on which converters can accept which arguments.

## New resolver system

* Resolver system should just use existing converter system (e.g. ExecConverter would do the same thing). Simply register a new ConversionGraph
- ~Generates faux type strings something like: `git:34fea34fe48db34,/some/path` -> `git:34fea34fe48db34` -> `file:/some/path`~
- Better system: `git_cloned` -> `git_updated` -> `git_file`
- ResolverConverters have a `detect_resolver_type

* All interior resources would be MutableResource, while the final would be ForeignResource

* Converter system has the following concepts built in:
- "understood arguments"
- If any converter in a conversion path understands an argument, that argument is propagated backward to that point
- For example: `thumb.png:500x500` gets normalized to `thumb.png:resolution=500x500` and for the STL->PNG conversion we'd have `STL` -> `PNG:resolution=500x500`

# Original notes

- The present TypeString argument system is not sufficient, it needs

- Arguments need to have type checking and association with converters

- They need to be carried through and be universally defined (for example, resolution would be understood by many converters)

- Converters need to supply hints as to what arguments they accept (and when a graph is drawn with types with arguments, it chooses paths that include converters)

Start by reading the existing converter system and the profile code referenced in the notes, then review the related issues #38 and #23. Trace how understood arguments, conversion paths, ResolverConverters, MutableResource, and ForeignResource currently fit together. Done requires an agreed argument and resolver design with clear converter associations, propagation rules, and path-selection behavior.

Written by the indexing model from the issue text.