test and fix yul codegen name conflicts

[sbillig]

We probably have name conflict issues in our yul codgen. For example, we use the user-supplied fn arg names, which might conflict with our v0, v1, .. var names.

fn foo(v0: u256) {
  let x = 1
  ..

probably becomes

function foo(v0) {
  let v0 := 1
  ..

There are almost certainly other possible name collision cases, eg if two fe functions have the same name but are in different modules, they (probably) end up in yul with the same name.

[sbillig]

gpt says:

• Module collisions (definitely broken): mangled_name uses only func.name as the base (crates/mir/
  src/monomorphize.rs:437), so mod a { fn foo } + mod b { fn foo } produces two MIR/Yul functions
  both named foo
• Type-name collisions inside generic mangling (definitely broken): generic suffixes/hash are
  based on ty.pretty_print() (crates/mir/src/monomorphize.rs:478) but pretty_print for ADTs is
  only the bare identifier (crates/hir/src/analysis/ty/ty_def.rs:1141), so two distinct types
  named S in different modules both mangle as S and collide
• Impl method prefix has the same problem: associated_prefix also uses ty.pretty_print() (crates/
  mir/src/monomorphize.rs:491), so inherent methods on same-named types in different modules can
  collide.
• Likely trait-edge collision: mangling doesn’t include “which trait” for trait methods; if two
  different traits define the same method name and you implement both for the same Self, you’re
  relying on type args alone to disambiguate, which can collide for identical arg strings.
• No collision detection: there’s no sanity check that instances[*].symbol_name is unique before
  codegen; codegen just emits mir_func.symbol_name directly (crates/codegen/src/yul/emitter/
  function.rs:57), so collisions become invalid Yul.
• Codegen name-based special case: try_collapse_cast_shim is purely string-pattern based (crates/
  codegen/src/yul/emitter/util.rs:20); a user-defined function named like __u32_as_u256 would get
  “optimized away” incorrectly.

[sbillig]

Easy improvement would be to prefix one set of names (user-provided or machine-generated) with $.