Asset relocator refactoring

[guybedford]

This implements the refactoring described in #223.

Binding analysis is now fully general with no specific import / require / binding cases.

It's a pretty major refactoring, but seems to have worked out well in the end luckily!

[guybedford]

(this is based to #232 so should be merged after #233).

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@208cdc0). Click here to learn what that means.
The diff coverage is 91.08%.

@@            Coverage Diff            @@
##             master     #234   +/-   ##
=========================================
  Coverage          ?   69.86%           
=========================================
  Files             ?       16           
  Lines             ?      657           
  Branches          ?        0           
=========================================
  Hits              ?      459           
  Misses            ?      198           
  Partials          ?        0

Impacted Files	Coverage Δ
src/utils/binary-locators.js	83.78% <83.78%> (ø)
src/loaders/relocate-loader.js	97.07% <93.1%> (ø)
src/utils/wrappers.js	93.93% <93.93%> (ø)

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