Route using map rather than fold to prevent state from parameters affecting the outer state

[rafaelbey]

What type of PR is this?

Bug Fix

What does this PR do / why is it needed ?

Currently, routing does a fold of the parameters of a function, and takes the output of that fold as the state to also route the outer function.

This has two side effect:

• The firsts parameters affect the routing of subsequent parameters
• The output of the fold affect the routing of the outer function

One visible outcome is when expressions have multiple from(), where these are not properly respected, leading to incorrect routing.

This PR also start treating let as a specialize function to ensure the platform generates the proper allocations nodes.

List of features this would enable:

• let statement routed thru platform to ensure allocation node is created regardless of assignee is routed:

let var = now()->from(runtime)

• let of relation expression and reference it on subsequent expressions:

let rel = #>{database}#->from(runtime);
$rel->filter(...);

• let of function with multiple expressions

function hello(): String[1]
{
 let a = 'hello';
 let b = 'world';
 $a + $b;
}

// to route/execute
let r = hello();
$r + '!';

• write with multiple from calls (cross-store)

#>{database}#->from(runtimeABC)->write(#>{anotherDatabase}#)->with(runtimeXYZ);

• join with multiple from calls (cross-store)

#>{database}#->from(runtimeABC)->join(#>{anotherDatabase}#->from(runtimeXYZ));

[github-actions]

Test Results

0 files   -   1 017  0 suites   - 1 017   0s ⏱️ - 1h 43m 21s
0 tests  - 12 824  0 ✔️  - 12 715  0 💤  - 109  0 ❌ ±0 
0 runs   - 18 828  0 ✔️  - 18 719  0 💤  - 109  0 ❌ ±0 

Results for commit 99feeda. ± Comparison against base commit d467d9a.

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