Fix: const_eval bugs (#4761)

## Description

This PR is necessary for #4708

It fixes some problems with `const_eval`, most importantly:

1 - It differentiates when an eval cannot be used inside const (there is
a new error for that), and when expressions/statements just do not
produce any value;
2 - It always cleans up `eval_codeblock` (it never shortcircuit with
`?`);

Another advantage of the explicit error is the exact span of the
"offending" expression, which we can use in the future for better error
messages.

## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

---------

Co-authored-by: Anton Trunov <[email protected]>

sway-core/Cargo.toml

@@ -14,10 +14,12 @@ derivative = "2.2.0"
 dirs = "3.0"
 either = "1.6"
 ethabi = { package = "fuel-ethabi", version = "18.0.0" }
-etk-asm = { package = "fuel-etk-asm", version = "0.3.1-dev", features = ["backtraces"] }
+etk-asm = { package = "fuel-etk-asm", version = "0.3.1-dev", features = [
+    "backtraces",
+] }
 etk-dasm = { package = "fuel-etk-dasm", version = "0.3.1-dev" }
 etk-ops = { package = "fuel-etk-ops", version = "0.3.1-dev" }
-fuel-abi-types = "0.1" 
+fuel-abi-types = "0.1"
 fuel-vm = { workspace = true, features = ["serde"] }
 hashbrown = "0.13.1"
 hex = { version = "0.4", optional = true }