[microcosm] Add remove data helper. Setting undefined removes props

[nhunzaker]

This commit updates the way set works such that it removes a property if it is undefined. It also adds an remove function, which is an alias for set(object, prop, undefined).

import { set, remove } from 'microcosm'

val records = { 1: { name: 'Billy' }, 2: { name: 'Sal' } }

// These are the same
set(records, 1, undefined) // { 2: { name: 'Sal' } }
remove(records, 1)         // { 2: { name: 'Sal' } }

I also took this as an opportunity to rework flow types and refactor some of the data operation code to avoid null checks.

[codecov-io]

Codecov Report

Merging #505 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #505      +/-   ##
==========================================
+ Coverage   99.69%   99.69%   +<.01%     
==========================================
  Files          27       27              
  Lines         658      659       +1     
  Branches      129      130       +1     
==========================================
+ Hits          656      657       +1     
  Misses          2        2

Impacted Files	Coverage Δ
packages/microcosm/src/data.js	100% <100%> (ø)	:arrow_up:

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