## Why this should be merged
The current rlpgen partially supports named types with basic underlying,
and it generates the rlp without correct conversion i.e:
`w.WriteUint64(obj.Uint64NewT)`
This PR adds the full support it
## How this works
Adds check for `isNamedWithBasicUnderlying` and then returns a `op` with
`makeNamedBasicOp` which is basically a `basicOp` but with the `typ`
refers to the named type so that it can correctly encode decode with the
named type.
## How this was tested
Added UT tests
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Signed-off-by: Ceyhun Onur <ceyhunonur54@gmail.com>