perf: skip reflect.ValueOf in Marshal() for common types

[mykaul]

Summary

• Adds a type-switch fast path at the top of Marshal() that skips reflect.ValueOf for common primitive types and their pointer variants
• The sub-marshalers already handle both pointer and non-pointer types natively (including nil pointer checks), making the reflect-based dereference+recurse path unnecessary for these types
• For pointer types the improvement is dramatic since the old path allocated an extra interface box via Elem().Interface()

Benchmarks

Type	Old (ns/op)	New (ns/op)	Change	Old allocs	New allocs
*int64	36.95	15.65	-57.65% (p=0.008)	2 / 16B	1 / 8B
*string	51.78	21.03	-59.39% (p=0.008)	2 / 32B	1 / 16B
bool	13.77	12.96	-5.88% (p=0.008)	1 / 1B	1 / 1B
time.Time	22.22	20.76	-6.57% (p=0.008)	1 / 8B	1 / 8B
geomean	19.95	15.45	-22.55%

For a query with 10 pointer parameters, this saves ~300ns and 10 allocations per execution.

Risk

Low — falls through to existing reflect path for any type not in the fast-path list. All unit tests pass with -race.

[mykaul]

CI Failures — unrelated to this PR

Both failures are known pre-existing flakes:

1. Cassandra 5-LATEST — TestConsistencySerial timed out after 120s. Known flaky Cassandra integration test tracked in Fix integration tests that are failing on cassandra #456.

2. Scylla LTS-PRIOR — TestSliceMapMapScanTupleTypes failed with "Another global topology request is ongoing, please retry." during TRUNCATE. Scylla-side topology contention flake (no open issue found in this repo).