Issue title: Improve docstrings and tests for utils inside misc.ts
 Issue body: This is the first in a series of issues related to improving the test coverage within the `core-utils` package.

It should be very easy to add some tests for the functions inside of [misc.ts](https://github.com/ethereum-optimism/optimism/blob/develop/packages/core-utils/src/common/misc.ts). These are all very simple functions. As part of this issue, I would also like to improve the docstrings for each function. See the docstring for [reqenv](https://github.com/ethereum-optimism/optimism/blob/eb926e2f90b5eec4558f3c82746b639b19ddf9d5/packages/core-utils/src/common/misc.ts#L22-L27) as an example of a good docstring. We need to add docstrings for [clone](https://github.com/ethereum-optimism/optimism/blob/eb926e2f90b5eec4558f3c82746b639b19ddf9d5/packages/core-utils/src/common/misc.ts#L15) and [getenv](https://github.com/ethereum-optimism/optimism/blob/eb926e2f90b5eec4558f3c82746b639b19ddf9d5/packages/core-utils/src/common/misc.ts#L36).
 Comments: 
 Comment 0: I'm giving this a go, will update if I get blocked.
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