Revert use of operator T(), etc and remove conflicting cast operator to unsigned from RLP.

unsigned is size_t on 32 bit builds, and the compiler therefore thinks conversion from RLP to vector<T> is ambiguous between:
RLP::operator vector<T>() const
and
vector<T>::vector<T>(size_t size)
Missing part of previous commit.
Merge remote-tracking branch 'upstream/develop' into develop

Conflicts:
	libdevcore/RLP.h