WIP: Add conditional support for gfs.promises.

[coreyfarrell]

This support is dependent on util.promisify existing, so node.js 8+.

Fixes #160

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@isaacs I think the code itself is complete, I want to seek feedback before I spend the time on more test coverage. Of note unlike native fs.promises, reading the gfs.promises property will never produce an 'experimental' warning. Additionally native fs.promises is not available in node.js 8 or 9, gfs.promises is available in those versions. The list of functions that are promisified is from Object.keys(require('fs').promises).sort() in node.js 12.7.0.

One difference, require('fs').promises appears to be separate wrappers around internal functions. So fs.promises.stat is not the result of util.promisify(fs.stat). Technically speaking the correct way to deal with this would be to reimplement all patched functions based directly on fs.promises but that seems like it would be a maintenance burden and would limit use of gfs.promises to the versions of node.js which provide fs.promises. I'm not sure if there is someone from node.js we should ask if this makes an actual difference in any cases or if the node.js implementation does the same thing as util.promisify but with better performance.

[coreyfarrell]

I should have spent more time in the documentation, it does appear that fs.promises has some differences that are not insignificant. I'm having second thoughts about the wisdom of implementing gfs.promises in this way. Might be better to instead provide this as gfs.promisified or just not do this at all. I'm thinking if gfs.promises is provided it should operate on FileHandle objects which would limit the feature to node.js 10+ and would not prevent the experimental warning from versions of node.js which provide it.

[coreyfarrell]

Abandoning for now