Vmtestsfix

[MrChico]

Unfortunately, I introduced a bug in f1b9b34, wherein the rlp encoding of transactions wrapped some fields twice. I.e. a transaction:

(Transaction {txData = "", txGasLimit = 0x61a80, txGasPrice = 0x1, txNonce = 0x0, txR = 0xe94818d1f3b0c69eb37720145a5ead7fbf6f8d80139dd53953b4a782301050a3, txS = 0x1fcf46908c01576715411be0857e30027d6be3250a3653f049b3ff8d74d2540c, txToAddr = Just 0x95e7baea6a6c7c4c2dfeb977efac326af552d87, txV = 0x1c, txValue = 0x186a0})

was encoded as:

[0x80,0x01,0x83061a80,0x94095e7baea6a6c7c4c2dfeb977efac326af552d87,0x830186a0,0x80]

instead of:

[0x,0x01,0x061a80,0x095e7baea6a6c7c4c2dfeb977efac326af552d87,0x0186a0,0x]

This error caused blockchain-tests to not be able to decode any "valid tests", since all transactions were malformed. But because InvalidTx was not treated as a fatal error, no failures were reported and we happily ignored all tests.

This fixes the tx encoding bug, and makes InvalidTx a fatal error.
Let's see how the ci does on this one

[MrChico]

I have also removed modexp from skipped tests, as its darwin problems were fixed by #250

[desaperados]

LGTM