global-context-scheme cleanup

[Erik Sandberg]

Hi,

The attached patch performs a number of cleanups:

1. ly:run-translator is split up into three separate functions.
2. get_output is eliminated; instead, score-level translators set a context 
property 'output in the score context.
3. (2) eliminates the need for the Score_translator and Score_context classes, 
so they are junked.

While (2) makes an improvement over the previous situation, I'm still not 
satisfied: I'd like to keep the output in translators, and then rewrite all 
uses of ly:run-translator to extract output from the translators instead. 

Other close items on the TODO are:
- Make translators keep track of parent/children without going through 
Context; eliminate Context::implementation
- Move lots of iteration-specific stuff from Context to Context_handle.

-- 
Erik

es-060708.diff
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