Preparation for intgeration of RefChecks

Added OverridingPairs
Small tweaks here and there.
Add showDcl method for SingleDenotations
Added utility method: underlyingSymbol
Initial versions of Variances and CheckVariances

Not yet integrated or tested.
Removed CheckTrees

It was unmaintained and superseded by TreeChecker. The only needed bit,
escapingRefs, got moved to Typer.
Two variance-related fixes in Desugar

1) Type parameter accessors inherit their variance from the type parameter
2) Copy method parameter defaults are annotated @uncheckedVariance. This is
necessary because default methods will be checked for variance.
Enabled variance checking

Variance checking is now run as part of type-checking.

Fixed tests that exhibited variance errors. Added tests where some classes of
variance errors should be detected.
Updated comment
Added assertion that casts never go to singleton types.

Disabled for now, awaiting a fix.
Merge pull request #147 from dotty-staging/transform/variance

transform/variance