internal/dinosql: Emit Querier interface

[kyleconroy]

Fixes #235

When emit_interface is true, output a Querier interface in the generated package. For the ondeck example, this interface is:

type Querier interface {
	CreateCity(ctx context.Context, arg CreateCityParams) (City, error)
	CreateVenue(ctx context.Context, arg CreateVenueParams) (int32, error)
	DeleteVenue(ctx context.Context, slug string) error
	GetCity(ctx context.Context, slug string) (City, error)
	GetVenue(ctx context.Context, arg GetVenueParams) (Venue, error)
	ListCities(ctx context.Context) ([]City, error)
	ListVenues(ctx context.Context, city string) ([]Venue, error)
	UpdateCityName(ctx context.Context, arg UpdateCityNameParams) error
	UpdateVenueName(ctx context.Context, arg UpdateVenueNameParams) (int32, error)
	VenueCountByCity(ctx context.Context) ([]VenueCountByCityRow, error)
}

[kyleconroy]

@myles-mcdonnell Is this what you're looking for?

[myles-mcdonnell]

Hi Kyle, yes exactly what I'm looking for, thanks.

Slight problem though, one of my queries includes a uuid and the emitted source doesn't include the import statement for github.com/google/uuid .. I had a look to see if I could find how to fix this and raise a PR but to no avail.