`gno run` should have an ExecContext

[thehowl]

similar to testing exec context, to support functions like std.GetHeight(). It currently doesn't:

gno/gnovm/cmd/gno/run.go

Lines 115 to 121 in 9897b66

	m := gno.NewMachineWithOptions(gno.MachineOptions{
	PkgPath: string(files[0].PkgName),
	Input: stdin,
	Output: stdout,
	Store: testStore,
	Debug: cfg.debug || cfg.debugAddr != "",
	})

cc @wyhaines

[wyhaines]

So....I have a branch that implements this. It is based on how it works for the 'test' command. I'll clean it up a bit and provide a PR.