Add optimized big vector functionality, helpers

[alnoki]

• Adds a size_of() function analogous to sizeof() in C.
• Adds a 4K disk sector optimized implementation of BigVector.
• Adds simple index-based borrowers/swap remove for BigVector.
• Adds tests for all of the above, byte size analysis
• Addresses #3695

Tagging @davidiw per #4051 , @gregnazario per recent discussion on vector size, @CapCap per recent discussion on struct field byte size, and @hariria , @lightmark , @wrwg , @zekun000, @runtian-zhou per #3695

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This change is