Use MPS_ARGS_BEGIN etc in the Scheme example and in the manual.
Use MPS_BEGIN and MPS_END instead of BEGIN and END.
The _FIELD macros don't need to be function-like macros.
Restore TODO comments in mps.h.
Scheme example compiles with -std=c89.
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Change: 181749
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Correct the documentation for MFS: constructor takes extend_by and unit_size in that order.
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Change: 180372
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