Adding some well-formedness checks.

* Prevents underscores in binding position inside graph.  Fixes #243
* Prevents underscores being used as vars
* Prevents underscores being used as shadowed ids
* Prevents duplicate fields in an object literal
Fix Set bugs caused by mixing constructors.
fixing the locations of underscore vars and shadows.  also filtering out underscore names in repl-support, since that happens before well-formedness gets to complain...
Subtle name-resolution bug in resolving the parameters of for-loops, and a regression test to make sure we don't make this mistake in the future.  Basically, the values being bound as the initial values to the looped function should be resolved in the environment outside the loop, while the names of the parameters are accumulated to provide the environment inside the loop.
makeMatch
checkpoint state --- not good yet b/c of problems with test-m-graph
special settings for bootstrap
bootstrap with broken but non-looping pretty-printing for data
good state of builtin torepr and _match with new raw fields access
Merge branch 'master' into ref
Merge branch 'ref' into torepr-pp
cleanup