Inline the serverless utils package

[GrahamCampbell]

Untested. Closes #147.

[mnapoli]

Happy to merge if it's tested manually :p

[G-Rath]

This is looking ok to me, though it's been a bit painful to try and fully test manually - if there's still concern, do we want to try splitting this up a bit e.g. inline one utility at a time.

Alternatively we could have each utility go through a shim-type function that has an env switch between the actual inlined version and the @serverless/utils version (though I'm a little loathed at that idea since we've been living with that for AWS SDK v3 for while now 😅)