core: reduce allocs in hot method: priceHeap.Less()

[jmank88]

I did some cpu profiling with pprof during a benchmark execution. Here is top:

      flat  flat%   sum%        cum   cum%
    82.36s 20.53% 20.53%     82.38s 20.53%  runtime.cgocall
    60.14s 14.99% 35.52%    124.78s 31.10%  github.com/gochain-io/gochain/core.priceHeap.Less
    29.45s  7.34% 42.86%     29.45s  7.34%  runtime.memmove
    26.44s  6.59% 49.45%     54.20s 13.51%  runtime.mallocgc
    14.87s  3.71% 53.16%     31.22s  7.78%  runtime.scanobject
    14.48s  3.61% 56.76%     14.48s  3.61%  runtime.duffcopy
    11.19s  2.79% 59.55%     18.60s  4.64%  runtime.mapaccess2
    10.57s  2.63% 62.19%     84.46s 21.05%  github.com/gochain-io/gochain/core/types.Sender
    10.05s  2.50% 64.69%     10.05s  2.50%  runtime.heapBitsForObject
     8.04s  2.00% 66.70%      8.24s  2.05%  syscall.Syscall

The priceHeap.Less method looked suspicious. It turns out it just compares two transactions' gas prices: h[i].GasPrice().Cmp(h[j].GasPrice()). Digging deeper, Transaction.GasPrice (reasonably) returns a copy of a big.Int, rather than its own (mutable) field value. Less is only inspecting, so we'd prefer direct access to the field. The new Transaction.CompareGasPrice method compares the unexported fields directly, reducing allocations from copying and heap garbage. BenchmarkTransaction_CompareGasPrice illustrates the difference:

BenchmarkTransaction_CompareGasPrice/copy-4         	10000000	       115 ns/op	      96 B/op	       2 allocs/op
BenchmarkTransaction_CompareGasPrice/nocopy-4       	50000000	        30.3 ns/op	       0 B/op	       0 allocs/op

This is a cpu profile after making the change:

      flat  flat%   sum%        cum   cum%
    61.18s 27.04% 27.04%     61.19s 27.05%  runtime.cgocall
    18.81s  8.31% 35.36%     35.04s 15.49%  math/big.(*Int).Cmp
    16.23s  7.17% 42.53%     16.23s  7.17%  math/big.nat.cmp
    15.29s  6.76% 49.29%     49.27s 21.78%  github.com/gochain-io/gochain/core/types.(*Transaction).CompareGasPrice
     9.62s  4.25% 53.54%      9.62s  4.25%  runtime.duffcopy
     7.70s  3.40% 56.94%     53.82s 23.79%  github.com/gochain-io/gochain/core/types.Sender
     7.32s  3.24% 60.18%     11.89s  5.26%  runtime.mapaccess2
     5.19s  2.29% 62.47%      5.34s  2.36%  syscall.Syscall
     4.92s  2.17% 64.65%     13.94s  6.16%  runtime.mallocgc
     4.27s  1.89% 66.54%     14.93s  6.60%  github.com/gochain-io/gochain/core/types.(*Transaction).Hash

Notably, CompareGasPrice is lower than Less was, and runtime.mallocgc dropped.

I assume that runtime.cgocall is leveldb, which we'd rather not mess with if we don't have to.