The position may be token.NoPos, an invalid position,
if there's a single result parameter without parentheses.
Using token.FileSet.Offset on token.NoPos+1 would panic:

	panic: invalid Pos value 1 (should be in [774665, 781950])

We didn't catch this because our tests always start with an empty
token.FileSet, so most of our files start with the base position 1.
A user ran into the bug when formatting many files at once.

To ensure we catch these bugs quicker in the future,
insert a dummy ten-byte file to every new FileSet we create.

Finally, add a test, which reproduces the panic thanks to the dummy base
file mentioned above.

Fixes #166.

For instance, don't group exported and unexported methods,
nor methods with non-methods.

Fixes #168.

For an input such as:

	var _ = map[string]string{
		/*
			joint comment
		*/
		"foo": "bar",
	}

We would incorrectly join the comment with the first element:

	var _ = map[string]string{
		/*
			joint comment
		*/ "foo": "bar",
	}

Just like in other parts of the codebase,
consider the first comment's position if it exists.

Fixes #170.

This way, users calling -s or -r get a helpful message.
The use of -s can still let the tool work, for now.

Fixes #174.

Used by some versions of GoLand and friends,
https://www.jetbrains.com/help/go/disabling-and-enabling-inspections.html

New versions use `//goland:noinspection` instead/additionally, but it
doesn't hurt much to support the older (and still the only documented)
one.