feat: Make it possible to build and run without jemalloc available

[hassila]

Description

Some platforms doesn't have jemalloc available and it'd be nice to be able to use benchmark without the malloc counters there.

Fixes #15 and Fixes #60 (sort of)

To run test suite on macOS without crash you can now use: BENCHMARK_DISABLE_JEMALLOC=1 swift test

To run benchmarks without jemalloc installed:

BENCHMARK_DISABLE_JEMALLOC=1 swift package benchmark

How Has This Been Tested?

Manually tested with jemalloc installed/uninstalled.

Minimal checklist:

• [x] I have performed a self-review of my own code
• [x] I have added DocC code-level documentation for any public interfaces exported by the package
• [x] I have added unit and/or integration tests that prove my fix is effective or that my feature works

[github-actions[bot]]

Pull request benchmark comparison [ubuntu-latest] with 'main' run at 2023-03-22T16:08:34+00:00 Pull request had performance regressions

[github-actions[bot]]

:tada: This PR is included in version 1.2.0 :tada:

The release is available on GitHub release

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