Minor fixes, changes to FrozenBitmap API (renaming)

[lemire]

• Removes unmaintained Makefile. The Makefile has obviously not been used by anyone for quite some time because its scripts were broken.
• Removes obsolete TestConcurrent* instructions, we have no such tests.
• Completing a few missing go fmt.
• The FrozenBitmap errors are renamed according to the standard which indicates that their names should start with Err. So we are changing our public API with respect naming which might cause some code brokerage, but it should be a minor inconvenience and the frozen API is for advanced users only.

There are other issues that go vet exposes, but this PR will not solve these. Pull requests are invited (@joenall), they are:

./roaring.go:130:19: method ReadFrom(reader io.Reader, cookieHeader ...byte) (p int64, err error) should have signature ReadFrom(io.Reader) (int64, error)
./serialization_littleendian.go:30:12: possible misuse of reflect.SliceHeader
./serialization_littleendian.go:37:38: possible misuse of reflect.SliceHeader
./serialization_littleendian.go:46:12: possible misuse of reflect.SliceHeader
./serialization_littleendian.go:53:38: possible misuse of reflect.SliceHeader
./serialization_littleendian.go:62:12: possible misuse of reflect.SliceHeader
./serialization_littleendian.go:69:38: possible misuse of reflect.SliceHeader

I do not understand these warnings, or rather I do not think that they indicate that something is actually wrong with the code. They are pedantic warnings. It is not immediately obvious what to do about those.

Fixes #405