src: separate out NgLibMemoryManagerBase
Extracted from the QUIC PR
So far, this is only used by the QUIC PR directly but the change itself is independent of QUIC, even if not used directly by anything else yet. Separated out per request.
/cc @addaleax @sam-github
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
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