Add option to lazy load the model

[gstvg]

Add method to lazy load the model, saving only
the offset at each word vector starts on the
io.Reader, and reading at it only when needed
Adds a flag lazy on word-server to use it
Also calls file.Close() on word-server in
case of error loading the model, because the
deferred one isn't called after os.Exit

[gstvg]

A bit of context, I'm using it on a serverless lambda web api (knative), the model is only queried a few times before the program exits. Reduces the cold-start a lot

[gstvg]

What should I do here?

[dhowden]

Hey! Thanks for the contribution - this is a great idea!

Reading through the change: instead of having the and lazy/original model inside Model, how about putting the lazy implementation in a new model implementation entirely:

type LazyModel struct {
	dim       int
	lazyWords map[string]int64
	reader    LazyReader
}

... and then implement the Coser methods on this new type.