Reduced big int allocation by making stack items modifiable. Instead of
adding items such as `common.Big0` to the stack, `new(big.Int)` is
added instead. One must expect that any item that is added to the stack
might change.
* Add params package with exported variables generated from
github.com/ethereum/common/blob/master/params.json
* Use params package variables in applicable places
* Add check for minimum gas limit in validation of block's gas limit
* Remove common/params.json from go-ethereum to avoid
outdated version of it
