perf: encoder / decoder of points, elements, should use bufio

[gbotrel]

Reading 32bytes per 32bytes a 6GB list of points from a os.File is terrible for perf.

Without changing the code, just wrapping the os.File io.Reader in to a bufio.Reader (for example bufio.NewReaderSize(f, fr.Bytes*(1<<10))) improves performance by a 10x factor.

Similar logic for io.Writer and bufio.Writer .

However our APIs work with Writer and Reader, and doing a bufio.Reader inside the functions may result in the function reading too many bytes. (calls to read functions are sometime encapsulated or following one another with the same Reader) .

Maybe we could add a high level ReadFromFile to avoid putting the burden of wrapping the reader on the caller;