Restrict space and turn to Player API

[olistic]

With these changes, it's no longer possible to call methods on the spaces nor on the turn (passed to the playTurn() function) 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().isEnemy()) {
      // Instant kill.
      const enemy = warrior.feel().getUnit();
      enemy.health = 1;
      warrior.attack();
    } else {
      warrior.walk();
    }
  }
}

Closes #71

WIP

[codecov-io]

Codecov Report

Merging #106 into master will increase coverage by 0.07%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #106      +/-   ##
=========================================
+ Coverage   90.83%   90.9%   +0.07%     
=========================================
  Files          80      79       -1     
  Lines        1015    1023       +8     
  Branches      159     160       +1     
=========================================
+ Hits          922     930       +8     
  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/look.js	100% <100%> (ø)	:arrow_up:
packages/warriorjs-core/src/Unit.js	100% <100%> (ø)	:arrow_up:
packages/warriorjs-abilities/src/listen.js	100% <100%> (ø)	:arrow_up:
packages/warriorjs-core/src/Position.js	100% <100%> (ø)	:arrow_up:
packages/warriorjs-abilities/src/feel.js	100% <100%> (ø)	:arrow_up:
packages/warriorjs-core/src/Space.js	100% <100%> (ø)	:arrow_up:

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[olistic]

Closing in favor of #114.