Rename toU64(), toU256() for consistency with other functions

[ImTei]

Description

In slow & fast VM, there are toU64(v uint8) and toU256(v uint8). But in the Solidity implementation, there are functions with the same name, but their argument type is uint64. This makes developer confused and could make bugs. So renamed go functions to byteToU64() and byteToU256() to prevent confusion.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.39024% with 64 lines in your changes missing coverage. Please review.

Project coverage is 61.63%. Comparing base (3f1f5fc) to head (429e8cd).

Files with missing lines	Patch %	Lines
rvgo/slow/vm.go	82.17%	24 Missing and 12 partials ⚠️
rvgo/fast/vm.go	81.45%	24 Missing and 4 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #119   +/-   ##
=======================================
  Coverage   61.63%   61.63%           
=======================================
  Files          27       27           
  Lines        4147     4147           
=======================================
  Hits         2556     2556           
  Misses       1444     1444           
  Partials      147      147           

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