refactor pvm memory

[qiweiii]

close #240

[codecov]

Codecov Report

Attention: Patch coverage is 87.14919% with 87 lines in your changes missing coverage. Please review.

Project coverage is 77.22%. Comparing base (667f5b3) to head (cb38eb8).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
PolkaVM/Sources/PolkaVM/Memory.swift	82.55%	71 Missing ⚠️
.../Blockchain/VMInvocations/HostCall/HostCalls.swift	0.00%	8 Missing ⚠️
PolkaVM/Sources/PolkaVM/VMState.swift	0.00%	5 Missing ⚠️
...s/Blockchain/VMInvocations/HostCall/HostCall.swift	0.00%	1 Missing ⚠️
PolkaVM/Sources/PolkaVM/Instruction.swift	0.00%	1 Missing ⚠️
PolkaVM/Sources/PolkaVM/StandardProgram.swift	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
+ Coverage   76.08%   77.22%   +1.14%     
==========================================
  Files         284      285       +1     
  Lines       22250    22740     +490     
==========================================
+ Hits        16928    17561     +633     
+ Misses       5322     5179     -143     

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