Range based loops

Range based loops were discussed several times, including at the Solidity Summit (#9054), in #4596, and in #9117.

Main motivation is improving code for analysis and reducing gas cost of checked array access.

This is a dedicated issue discussing the syntax and semantics:
```solidity
uint[] array;

for (uint value: array) {}

// And with slicing
for (uint value: array[start:end]) {}
```

Of course for value types this doesn't support references, but references/pointers is probably something we should solve separately in general.

Review the prior discussions in #9054, #4596, and #9117 before attempting implementation. Define the syntax and semantics for range-based loops and array slicing, including the stated value-type limitation; the work is done when the language behavior and implementation scope are agreed and validated.

Written by the indexing model from the issue text.