Add VFSFind so a VFS can delegate temp files to the base VFS

[jtarchie]

Closes #15.

Problem

When a query is too big to sort in memory, SQLite opens a temporary file through the same VFS that serves the database. A read-only VFS (compressed, remote, etc.) can't create files, so the open fails and the whole query fails with unable to open database file. The only workaround was making every caller run PRAGMA temp_store = memory.

Fix

Adds VFSFind(name string) VFS (the API sketched in #15). It looks up a VFS SQLite already has registered — pass "" for the default one — and returns it as a VFS that Go code can call. A read-only VFS can then forward temp files (SQLite asks for them with an empty file name) to a VFS that can create them, the same way SQLite's own appendvfs does:

func (vfs *MyVFS) Open(name string, flags sqlite3vfs.OpenFlag) (sqlite3vfs.File, sqlite3vfs.OpenFlag, error) {
	if name == "" { // temp file
		return vfs.base.Open(name, flags)
	}
	// ... open the main database yourself
}

Test

TestDelegateTempFiles runs a query that must spill, against a VFS that can't make temp files. Without a base VFS it fails (the bug from #15); delegating to VFSFind("") it passes.