Removes the base_uint constructor that took a string. Replaces
that functionality with two free functions named from_hex_text<>.
Use of from_hex_text<> looks like this:
auto v = from_hex_text<uint256>("AAA555");
static_assert (std::is_same<decltype(v), uint256>::value, "Huh!");
from_hex_text<> only operates on base_uint types. At the moment the
list of those types include:
o uint128,
o uint160,
o uint256,
o Directory,
o Account,
o Currency, and
o NodeID.
Using from_hex_text<> with any other types will not compile due to
an enable_if.
4515ac0bca