fix: avoid deprecated implicit Compare promotion for MoonBit v0.10.9

[brickfrog]

derive(Compare) generates a trait impl, and v0.10.9 deprecated the implicit promotion of derived trait methods to type methods. The internal comparison helpers called self.compare(other), which relied on exactly that promotion, so the build emitted 18 [0020] deprecation warnings across Date, Time, and DateTime — is_before, is_after, min, max, and both clamp bounds on each.

Note that moon check stayed silent; only the moon test build path reported them.

Change

Every site had the shape self.compare(x) OP 0, so use the comparison operators directly:

self.compare(other) < 0   ->  self < other
if self.compare(lo) < 0   ->  if self < lo

They desugar through the Compare impl without any method promotion, and read better than the explicit Compare::compare(self, other) OP 0 form the compiler suggests.

Deliberately not adding extend T with Compare::{compare, ..}: the generated interface exports derive(Compare, ...) and no compare method, so an extend declaration would widen the public API rather than fix the deprecation.

Verification

• src/pkg.generated.mbti byte-identical after moon info — no visible change for downstream users.
• 297/297 tests pass on wasm, wasm-gc, js, and native.
• Warning count in the full moon test --target all build: 0.
• moon fmt produced no further changes.

Checked under moonc v0.10.10+f8a486b6f (upgraded from v0.10.7+bc794d341). No other surface in the 0.10.7 -> 0.10.10 jump touches this repo: no defer/errdefer, no fragile_catch_all or guard_inexhaustive hits, StringBuilder() already the constructor form, and no use of @immut/hashmap/hashset/vector, bigint, @fs, Iter2 Show, from_sorted_array, or lexmatch/lexscan.