fix: Fixing external benchmarks memory problems

[Rumata888]

Description

Charlie's memory management updates made it so we can't allocate some of our structures in global scope, which broke the benchmarks. This is a fix
Resolves #548

Checklist:

• I have reviewed my diff in github, line by line.
• Every change is related to the PR description.
• The branch has been merged with/rebased against the head of its merge target.
• There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
• There are no circuit changes, OR a cryptographer has been assigned for review.
• New functions, classes, etc. have been documented according to the doxygen comment format. Classes and structs must have @brief describing the intended functionality.
• If existing code has been modified, such documentation has been added or updated.
• No superfluous include directives have been added.
• I have linked to any issue(s) it resolves.
• I'm happy for the PR to be merged at the reviewer's next convenience.

[ludamad]

Note AztecProtocol/aztec-packages#1174
Although as this continues to work in barretenberg repo I am taking a bit before looking into it again

[Rumata888]

Merged int aztec-packages