Move fantasy-land methods to the prototypes [PR-ready]

[cAttte]

currently, each class exposes its fantasy-land/ adapter methods by assigning to them in the constructor:

purify/src/Maybe.ts

Lines 434 to 441 in 9fffe0d

	'fantasy-land/equals' = this.equals
	'fantasy-land/map' = this.map
	'fantasy-land/ap' = this.ap
	'fantasy-land/alt' = this.alt
	'fantasy-land/chain' = this.chain
	'fantasy-land/reduce' = this.reduce
	'fantasy-land/extend' = this.extend
	'fantasy-land/filter' = this.filter

this isn't great because (a) it's a little memory-inefficient and (b) it's just ugly and/or out of place, for example, when enumerating an object's properties (serialization, debugging, etc.). a way to fix this is by assigning these methods in the prototype instead:

Nothing.prototype['fantasy-land/equals'] = Nothing.prototype.equals;
Nothing.prototype['fantasy-land/map'] = Nothing.prototype.map;
Nothing.prototype['fantasy-land/ap'] = Nothing.prototype.ap;
Nothing.prototype['fantasy-land/alt'] = Nothing.prototype.alt;
Nothing.prototype['fantasy-land/chain'] = Nothing.prototype.chain;
Nothing.prototype['fantasy-land/reduce'] = Nothing.prototype.reduce;
Nothing.prototype['fantasy-land/extend'] = Nothing.prototype.extend;
Nothing.prototype['fantasy-land/filter'] = Nothing.prototype.filter;

i'm down to make a PR, so just let me know