The generic pointer decoder did not advance the input position
for empty values. This can lead to strange issues and even
infinite loops.

The encoder was missing a special case for one element strings whose
element is below 0x7f. Such strings must be encoded as a single byte
without a string header.

Über-convenience.

All other pointer types can handle nil just fine.

This needs to be supported because []someInterface does occur sometimes.

Funny enough, the fix involves changes to the decoder. makeDecoder
cannot return an error for non-empty interfaces anymore because the type
cache builds both decoder and writer. Do the check at 'runtime' instead.

I'm reasonably confident that the encoding matches the output of
ethutil.Encode for values that it supports. Some of the tests have been
adpated from the Ethereum testing repository.

There are still TODOs in the code.

Decode error messages now say "expected input list for foo.MyStruct"
instead of just "expected List".

The documentation for reflect.Value.Index states that it will
panic for out-of-bounds indices. Since go 1.4, it actually panics.

There is no agreement on how to encode negative integers
across implementations. cpp-ethereum doesn't support them either.