Tolk v0.8: preparation for structures

A new version of Tolk was released several days ago. We're starting a way to eventually implement structures with auto packing to/from cells. This will take several steps (each publicly released), it's the first one.

✅ Notable changes in Tolk v0.8:
1. Syntax tensorVar.0 and tupleVar.0, both for reading and writing
2. Allow cell, slice, etc. to be valid identifiers

PR on GitHub with detailed info.

Using syntax `tensorVar.{i}` and `tupleVar.{i}`, you can access tensors/tuples by indices without unpacking them.

It works for tensors:

var t = (5, someSlice, someBuilder);   // 3 stack slots
t.0                     // 5
t.0 = 10;               // t is now (10, ...)
t.0 += 1;               // t is now (11, ...)
increment(mutate t.0);  // t is now (12, ...)
t.0.increment();        // t is now (13, ...)

t.1         // slice
t.100500    // compilation error

It works for tuples (does asm INDEX/SETINDEX under the hood):

var t = [5, someSlice, someBuilder];   // 1 tuple on a stack with 3 items
t.0                     // "0 INDEX", reads 5
t.0 = 10;               // "0 SETINDEX", t is now [10, ...]
t.0 += 1;               // "0 INDEX" to read 10, "0 SETINDEX" to write 11
increment(mutate t.0);  // also, the same way
t.0.increment();        // also, the same way

t.1         // "1 INDEX", it's slice
t.100500    // compilation error

It works for nesting var.{i}.{j}. It works for nested tensors, nested tuples, tuples nested into tensors. It works for mutate. It works for globals.

Why is this essential?

In the future, we'll have structures, declared like this:

struct User {
    id: int;
    name: slice;
}

Structures will be stored like tensors on a stack:

var u: User = { id: 5, name: "" };
// u is actually 2 slots on a stack, the same as
var u: (int, slice) = (5, "");

fun getUser(): User { ... }
// on a stack, the same as
fun getUser(): (int, slice) { ... }

It means, that `obj.{field}` is exactly the same as `tensorVar.{i}`:

var u: User = ...;   // u: (int, slice) = ...
u.id;                // u.0
u.id = 10;           // u.0 = 10

Same goes for nested objects:

struct Storage {
    lastUpdated: int;
    owner: User;
}

s.lastUpdated        // s.0
s.owner.id           // s.1.0

So, implementing indexed access for tensors/tuples covering all scenarios is a direct step towards structures.