raco: support an AppendTemp operator

AppendTemp is like StoreTemp except that it appends tuples. For now, at
least on the Myria backend, we only support Appending to temporary
relations so that the query-level fault tolerance issues are elided.

Also add a rule to Myrialang to map StoreTemp(UnionAll(ScanTemp, ..)) to
an AppendTemp on the final Myria physical plans.
query_tests: trying to TDD a bug in a real query, found diff bug
query_tests: fix style in prev tdd commit
expression: implement repr and str for Unbox
replace_with_repr: print bad representation on SyntaxError too
flake
query_tests: name output vars
more uda tests, still cannot trigger my issue
messed up a test:
tdd commits of more bugs
Add helper function for compiling away unbox operations in state variables
Push unboxing into distributed state variables
Add Expression methods to dummy aggregate outputs
Fix type name
Test of nary distributed UDA
Merge pull request #335 from uwescience/store-temp-append

raco: support an AppendTemp operator
Only associate decomposable state with first aggregate of a tuple
Delete unused functionality
Nasty shallow copy bug during function expansion
style fixes