Allow to have fixed-width integer types in the autogenerator

[fyfyrchik]

Is your feature request related to a problem? Please describe.

Sometimes I want to use uint32 instead of big.Int in the auto-generated wrappers.
Currently, I need to add another struct.

Describe the solution you'd like

pkg/rpcclient/unwrap has a beautiful unwrapper

neo-go/pkg/rpcclient/unwrap/unwrap.go

Line 98 in 6d20772

func LimitedInt64(r *result.Invoke, err error, minI int64, maxI int64) (int64, error) {

Allow to use it from inside the autogenerator.
A possible value to use for a type in rpcbinding configuration is Integer:int64. Another option is base: Integer; Name: int64.
We may also have other types (e.g. uint32), the details can be discussed. E.g. have LimitedInt[T interface { ~int* }] and LimitedUint[T interface{ ~uint* }] in unwrap and reuse them.

Describe alternatives you've considered

Provide custom type/unmarshaler defined outside the generated code. With it, we can have SDK types reused in the autogenerated code for contracts.

[roman-khimov]

This will be possible with NEP-25 implementation, it has a notion of type width.