Allow underscores as unused variable identifiers

[Gusarich]

Closes #319

• [x] I have updated CHANGELOG.md
• [x] I have added tests to demonstrate the contribution is correctly implemented: this usually includes both positive and negative tests, showing the happy path(s) and featuring intentionally broken cases
• [x] I have run all the tests locally and no test failure was reported
• [x] I did not do unrelated and/or undiscussed refactorings

[novusnota]

As of now, it only works for foreach and catch, right?

[Gusarich]

As of now, it only works for foreach and catch, right?

Are there any other cases where we need it?

[novusnota]

Are there any other cases where we need it?

I don't think so, at least now. When we'll have #298 in the future, identifiers in let declarations would need them though.

[anton-trunov]

When we'll have https://github.com/tact-lang/tact/issues/298 in the future, identifiers in let declarations would need them though.

Actually, let-statements should have underscores even now. This can be used as a clear indication that some function gets called not for its return value but for its side-effects:

let _ : Foo = effectfulFun(x, y, z);

When we have a bit of type inference, the snippet above will become

let _ = effectfulFun(x, y, z);

In this case a static analyzer won't complain about unused values, whereas it should issue a warning for

effectfulFun(x, y, z);

if effectfulFun is a non-void function.

[Gusarich]

Added support for let-statements and also resolved all merge conflicts

[Gusarich]

resolved merge conflicts

[Gusarich]

@anton-trunov resolved merge conflicts yet again after merge of #198

[Gusarich]

@anton-trunov have a look