rlp: add SplitUint64

[vivijj]

This can be useful when working with raw RLP data. Rules around integers in RLP are too complicated to just use SplitString and decode the resulting byte slice.

[codecov]

Codecov Report

Merging #770 into master will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #770      +/-   ##
==========================================
+ Coverage   50.45%   50.53%   +0.07%     
==========================================
  Files         410      410              
  Lines       53443    53461      +18     
==========================================
+ Hits        26965    27016      +51     
+ Misses      24421    24379      -42     
- Partials     2057     2066       +9     

Impacted Files	Coverage Δ
rlp/raw.go	95.14% <100.00%> (+4.55%)	⬆️
whisper/whisperv6/whisper.go	54.00% <0.00%> (-1.22%)	⬇️
p2p/rlpx.go	69.97% <0.00%> (-1.14%)	⬇️
rpc/handler.go	89.76% <0.00%> (-0.94%)	⬇️
p2p/simulations/mocker.go	34.54% <0.00%> (-0.91%)	⬇️
p2p/simulations/network.go	59.92% <0.00%> (-0.72%)	⬇️
trie/proof.go	76.11% <0.00%> (-0.38%)	⬇️
ctxc/downloader/downloader.go	78.15% <0.00%> (-0.33%)	⬇️
p2p/discover/v5_udp.go	73.31% <0.00%> (+0.46%)	⬆️
p2p/peer.go	79.90% <0.00%> (+0.91%)	⬆️
... and 5 more