Rename InstHead to InstRule and DeclOrInstHead to InstHead. Work on #31.
HLint fixes.

No functional changes. Mostly removing redundant parentheses/$.
Second round of HLint fixes.

No functional changes. Mostly removing redundant do/parentheses/$.
Add tests for the PackageImports extension.

Include positive (works when PackageImports is present) and negative
(compile error if PackageImports is absent) tests.
No code HSE code changes required to make these tests pass.