feat: support key re-use from serialized path

[stephenplusplus]

Fixes #59

This exposes a serialized property on a Key object. It will correctly differentiate between a key name and id, so that you can re-create a key with it:

const key = datastore.key({
  namespace: 'My-NS',
  path: ['Company', 123]
);
// Later...
const key = datastore.key(key.serialized);

[codecov]

Codecov Report

Merging #474 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #474      +/-   ##
==========================================
+ Coverage   98.24%   98.25%   +0.01%     
==========================================
  Files           5        5              
  Lines         684      689       +5     
  Branches      159      161       +2     
==========================================
+ Hits          672      677       +5     
  Partials       12       12

Impacted Files	Coverage Δ
src/entity.ts	98.59% <100%> (+0.02%)	⬆️

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[JustinBeckwith]

Why use Object.defineProperty as opposed to a regular getter?

[stephenplusplus]

Give it a few years-- Object.defineProperty will be in style again, and the kids will be wearing it on their hoodies.