Fix transitions of Microstate Arrays

[taras]

One of the features of Microstates is the ability to organize data into arrays. When you create an array microstate, you can use special transitions to manipulate this array in immutable way. For example, create([Number], [1, 2, 3]).push(4) will return a new array with item 4 added to the end.

Previously, we implemented map, filter, push, pop and others, but they were not implemented correctly. When you used one of these operations, the created microstate did not have a properly formed data structure. This PR fixes this problem allowing all of the supported transitions to be used.

Notable Changes

Removed splice

Previously, splice was used to implement most of the array transitions. This was removed and so was splice. We can re-add it if the need arises.

map microstates

Previously, map would call the mapping function with state for each element of the array. This was necessary because there was no easy way to get the state of each individual array item. Unfortunately, this also meant that there was no obvious way to perform microstate transitions inside of map operations. There are several issues effected by this #76 and #75.

Fortunately, #113 branch introduced the ability to get state for each composed microstate. This means that we're able to get state inside of map and allow transitions to happen. In this PR, I completed the change to allow performing transitions inside of map.

You can call transitions,

let dataset = create(Dataset, { records: [
  {content: 'Herro'},
  {content: 'Sweet'},
  {content: "Woooo"}
]});

let d2 = dataset.records.map(record => record.content.concat('!!!'));

d2.valueOf();
// { records: [
//  {content: 'Herro!!!'},
//  {content: 'Sweet!!!'},
//  {content: "Woooo!!!"}
// ]}

Or you can change the type by returning a new microstate.

class SweetSweetRecord extends Record {}

mapped = dataset.records.map(record => {
  if (record.content.state === 'Sweet') {
    return create(SweetSweetRecord, record);
  } else {
    return record;
 }
});

added clear transition

clear transition is equivalent to calling .set([])

Closes #125, #75 & #76

[coveralls]

Coverage decreased (-0.1%) to 90.37% when pulling f398cc2 on tm/array-graft into 56e27ad on master.