fn and lambda should support destructuring

**Describe the bug**
Currently when passing an array of tuple to a `map` with lambda, we can't destructuring the element into a tuple.

**To Reproduce**
```
fn muladd(a: u8, b: u8, c: u8) -> u8 {
let p = [(a, b, c)];
map(p, |(a, b, c)| {
a * b + c
})[0]
}
```
0003: fn muladd(a: u8, b: u8, c: u8) -> u8 {
0004: let p = [(a, b, c)];
0005: map(p, |(a, b, c)| {
~~~~~~~~~~~~~~~~~~^ ParseError: Expected 'identifier', got '(': Expected name (definition)
0006: a * b + c
0007: })
```

**Workaround**

Deconstruct the tuple explicitly within the lambda body:
```
fn muladd(a: u8, b: u8, c: u8) -> u8 {
let p = [(a, b, c)];
map(p, |t| {
let (a, b, c) = t;
a * b + c
})[0]
}
```

**Expected behavior**
Tuple can be deconstructed within lambda parameter declaration.

Start with the reported reproducer and its parser diagnostic, then trace how lambda parameter declarations and tuple destructuring are parsed. Verify the expected behavior by compiling the example and confirming it produces the same result as the explicit-deconstruction workaround.

Written by the indexing model from the issue text.