feat: Maps page update

[novusnota]

Added or refined:

• .get()
• .set()
• .asCell()
• Deletion of individual or all entries
• Iteration over maps
• Mention of issues with large maps and how to mitigate them:
  1. With upper-bounds
  2. Or with contract sharding

And everything with examples of Tact code, except for contract sharding, which will be added later as per #155.

As a bonus, added two neat examples of using maps as arrays to Cookbook (with handy comments): one from @howardpen9 :fire:, another from tact-by-example and @talkol :fire:

Closes #135 Closes #133

[novusnota]

Left to do:

And the same for arrPop() function just introduces.

[novusnota]

@anton-trunov It seems like the gas usage with packing the map into structure with methods (with one persistent state variable array: Array, see below) is the same as in the previous example (with using two: arr and arrLength). Or this is the case for the Sandbox.

Code of the new example (works, tested :)

```tact import "@stdlib/deploy"; struct Array { map: map; // «array» of Int values as a map length: Int = 0; // length of the «array», defaults to 0 } extends mutates fun push(self: Array, item: Int) { self.map.set(self.length, item); self.length = self.length + 1; } extends mutates fun pop(self: Array): Int { require(self.length > 0, "No items in the array to delete!"); // Note, that we use !! operator as we know for sure that the value would be there let lastItem: Int = self.map.get(self.length - 1)!!; self.map.set(self.length - 1, null); // delete the entry self.length = self.length - 1; // decrease the length return lastItem; } extends fun getLast(self: Array): Int { require(self.length > 0, "No items in the array!"); // Note, that we use !! operator as we know for sure that the value would be there return self.map.get(self.length - 1)!!; } contract MapsAsArrays with Deployable { // Persistent state variables array: Array; // structure defined above // Constructor (initialization) function of the contract init() { self.array = Array{map: emptyMap()}; // length defaults to 0 } // Internal message receiver, which responds to a String message "push" receive("push") { // Add a new item and increase the counter self.array.push(42); } // Internal message receiver, which responds to a String message "pop" receive("pop") { // Remove the last item and decrease the counter self.array.pop(); } // Internal message receiver, which responds to a String message "get_last" receive("get_last") { // Reply back with the latest item in the map if it exists, or raise an error self.reply(self.array.getLast().toCoinsString().asComment()); } // Getter function for obtaining the «array» get fun map(): map { return self.array.map; } // Getter function for obtaining the current length of the «array» get fun length(): Int { return self.array.length; } } ```

Added that example to the Cookbook + added a note discussing potential issues of the current one.

[novusnota]

Ready for review. Added more examples and refined previous ones — Map emulating: Arrays, Stacks, CircularBuffers.