Enforce Player API

[olistic]

With these changes, it's no longer possible to call methods on the spaces, units, or turn, that don't belong to the Player API.

Before this PR, a player could cheat by writing stuff like this:

class Player {
  playTurn(warrior) {
    if (warrior.feel().isUnit()) {
      const unit = warrior.feel().getUnit();
      unit.health = 1;
      warrior.attack(); // Instant kill!
    } else {
      warrior.walk();
    }
  }
}

Closes #71

[codecov-io]

Codecov Report

Merging #114 into master will increase coverage by 0.05%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
+ Coverage   90.76%   90.81%   +0.05%     
==========================================
  Files          80       79       -1     
  Lines        1007     1013       +6     
  Branches      156      157       +1     
==========================================
+ Hits          914      920       +6     
  Misses         77       77              
  Partials       16       16

Impacted Files	Coverage Δ
packages/warriorjs-core/src/Warrior.js	100% <ø> (ø)	:arrow_up:
packages/warriorjs-abilities/src/listen.js	100% <100%> (ø)	:arrow_up:
packages/warriorjs-units/src/Sludge.js	100% <100%> (ø)	:arrow_up:
packages/warriorjs-core/src/Unit.js	100% <100%> (ø)	:arrow_up:
packages/warriorjs-abilities/src/look.js	100% <100%> (ø)	:arrow_up:
packages/warriorjs-core/src/Position.js	100% <100%> (ø)	:arrow_up:
packages/warriorjs-core/src/Space.js	100% <100%> (ø)	:arrow_up:
packages/warriorjs-units/src/Archer.js	100% <100%> (ø)	:arrow_up:
packages/warriorjs-abilities/src/feel.js	100% <100%> (ø)	:arrow_up:
packages/warriorjs-core/src/Level.js	100% <100%> (ø)	:arrow_up:
... and 4 more

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