[Beta] Backports (#7945)

* ECIP 1041 - Remove Difficulty Bomb (#7905)

Enable difficulty bomb defusion at block:
 - 5900000 on Ethereum Classic mainnet,
 - 2300000 on morden testnet.

Reference:
https://github.com/ethereumproject/ECIPs/blob/master/ECIPs/ECIP-1041.md

* spec: Validate required divisor fields are not 0 (#7933)

* Add validate_non_zero function

It's used to validate that a Spec's uint field used as a divisor is not zero.

* Add deserialize_with to gas_limit_bound_divisor

Prevents panics due to divide-by-zero on the gas_limit_bound_divisor
field.

* Add deserialize_with to difficulty_bound_divisor

Prevents panics due to divide-by-zero on the difficulty_bound_divisor
field.

* Add validate_optional_non_zero function

Used to validate Option<Uint> divisor fields.

* Use deserialize_with on optional divisor fields.

* Add #[serde(default)] attribute to divisor fields

When using `#[serde(deserialize_with)]`, `#[serde(default)]` must be specified so that missing
fields can be deserialized with the deserializer for `None`.

* Kovan WASM fork code (#7849)

* kovan fork code

* introduce ethcore level vm_factory and let it fail

* fix json tests

* wasmcosts as option

* review changes

* wasm costs in parser

* fix evm tests

* review fixes

* fix test

* remove redundant json field

ethcore/src/json_tests/executive.rs

@@ -259,7 +259,7 @@ fn do_json_test_for(vm_type: &VMType, json_data: &[u8]) -> Vec<String> {
 				&mut tracer,
 				&mut vm_tracer,
 			));
-			let mut evm = vm_factory.create(params.gas);
+			let mut evm = vm_factory.create(&params, &machine.schedule(0u64.into()));
 			let res = evm.exec(params, &mut ex);
 			// a return in finalize will not alter callcreates
 			let callcreates = ex.callcreates.clone();