Reorganize stdlib

[nrfulton]

mellea/
  core/
    __init__.py
    base.py
    sampling.py
    backend.py
    formatter.py
  formatters/
      __init__.py <- exports main stuff.
  backends/
    adapters/
        __init__.py <- exports main stuff.
    openai.py
    litellm.py
    ollama.py
    move types.py to model_options.py 
    # other backends probably get moved into contribs or dedicated packages?
  stdlib/
    components/ <- most of what's in stdlib now goes here.
      __init__.py <- exports the components; mellea.stdlib.instruction.Instruction feels annoying.
      instruction.py
      requirement.py
      genslot.py
      document.py moved from base.py
      simple.py moved from base.py
      ... etc. ...
    samplers/
    reqs/
    tools/
  templates/ <- works as does now.
  helpers/ <- works as does now.

The contribs package then follows a similar structure to stdlib:

mellea-contribs/
  lib/
    components/
    samplers/
    reqs/
    tools/
    misc/ <- catch-all for other stuff
  backends/

Accumulated Proposed changes

• ModelOutputThunk._context should be the actual context not a list.
• ...