* core: Fix for consensus test gasLimit > 2^63-1 https://github.com/ethereum/tests/blob/develop/BlockchainTests/bcInvalidHeaderTest.json#L238

* core: fix testcase for uncle gasUsage > gasLimit : https://github.com/ethereum/tests/blob/develop/BlockchainTests/EIP150/bcUncleHeaderValiditiy.json#L986

* math/big: rename TTM63m1 -> MaxBig63, + go fmt

* core: documentation