eth: fix debug.storageRangeAt return value

[mexskican]

This PR Fixes #15196

• debug.storageRangeAt uses the rlp.Split function to return the content of the RLP values.
• Modify the test function in the case Split returns an error.

To test this PR I deployed on Rinkeby this contract:

contract test {
    uint v = 128;
}

The modification can be checked with this call:

> debug.storageRangeAt("0xd1a9569e5b5a482db6ac6c2a9fc9eddc58f49be1b8c4bff7c638239993708ceb", 0, "0xb9f998b90b2bfe93f0f34a2db45c3f70938d4fb7", "0x", 1)

It should return 0x80 instead of 0x8180 before.

[holiman]

Could you check (with the fix) the values returned for empty value?

[mexskican]

With the fix an empty value returns an empty storage array:

contract Test {
    string name = "";
}

Deployed on Rinkeby at : 0x649167b3818bd0a017f84a6321ea6b1755595aa5

> debug.storageRangeAt("0xa30c48206acf4da0e770df44d9f5a3ff59c742ede5362d3e2035ab2ce46c795c", 0, "0x649167b3818bd0a017f84a6321ea6b1755595aa5", "0x", 1)
{
  nextKey: null,
  storage: {}
}

This is the expected behavior right?

[holiman]

LGTM, but @fjl please verify

[mexskican]

ping @fjl