Use model struct as method parameter #872

Some of insert queries contains all columns of table as input parameters.
In that case generated method may apply model struct instead of ...Params struct.

For example following method
https://github.com/kyleconroy/sqlc/blob/bda1a0089e042a33f64f9dd1b59a2c3631fb659e/examples/ondeck/postgresql/city.sql.go#L28
may easily become

func (q *Queries) CreateCity(ctx context.Context, arg City) (City, error) {

Because table contains two columns
https://github.com/kyleconroy/sqlc/blob/bda1a0089e042a33f64f9dd1b59a2c3631fb659e/examples/ondeck/postgresql/schema/0001_city.sql#L1-L4
and the query uses all of them
https://github.com/kyleconroy/sqlc/blob/bda1a0089e042a33f64f9dd1b59a2c3631fb659e/examples/ondeck/postgresql/query/city.sql#L15-L21

Implementation

For output parameters there is struct "deduplication"
https://github.com/kyleconroy/sqlc/blob/bda1a0089e042a33f64f9dd1b59a2c3631fb659e/internal/codegen/golang/result.go#L183

I believe we need to introduce the same deduplication for input parameters
https://github.com/kyleconroy/sqlc/blob/bda1a0089e042a33f64f9dd1b59a2c3631fb659e/internal/codegen/golang/result.go#L166