add QueryBinary, an alloc-free way to read all rows into a buffer

[bradfitz]

WIP; goal is alloc-free reads of a query into a Go-provided buffer. Go code can then parse the simple binary format and alloc if needed (doing its own cache lookups, including alloc-free m[string([]byte)] lookups, and returning existing Views if data is unmodified)

(Counterproposal to #43)