Improve Any stack usage

By doing this unsafe approach we are able to save one call to morestack.

Stack usage

SUBQ $0x118, SP       // 280 bytes

Benchmark before
goos: linux
goarch: amd64
pkg: go.uber.org/zap
cpu: AMD EPYC 7B13
BenchmarkAny/normal             18570822                61.33 ns/op           24 B/op          1 allocs/op
BenchmarkAny/normal-2           20671512                59.85 ns/op           24 B/op          1 allocs/op
BenchmarkAny/normal-4           20064495                58.73 ns/op           24 B/op          1 allocs/op
BenchmarkAny/normal-8           19763770                60.18 ns/op           24 B/op          1 allocs/op
BenchmarkAny/normal_with_logger                  7673887               157.4 ns/op           112 B/op          3 allocs/op
BenchmarkAny/normal_with_logger-2                8318539               143.0 ns/op           112 B/op          3 allocs/op
BenchmarkAny/normal_with_logger-4                8584477               139.0 ns/op           112 B/op          3 allocs/op
BenchmarkAny/normal_with_logger-8                8203886               147.7 ns/op           112 B/op          3 allocs/op
BenchmarkAny/normal_new_goroutine                 273378              3873 ns/op             137 B/op          2 allocs/op
BenchmarkAny/normal_new_goroutine-2               496189              2141 ns/op              72 B/op          2 allocs/op
BenchmarkAny/normal_new_goroutine-4              1000000              1088 ns/op              72 B/op          2 allocs/op
BenchmarkAny/normal_new_goroutine-8              1743856               678.5 ns/op            72 B/op          2 allocs/op
PASS
ok      go.uber.org/zap 15.922s

Benchmark after
goos: linux
goarch: amd64
pkg: go.uber.org/zap
cpu: AMD EPYC 7B13
BenchmarkAny/normal             15874744                75.15 ns/op           24 B/op          1 allocs/op
BenchmarkAny/normal-2           16086948                71.64 ns/op           24 B/op          1 allocs/op
BenchmarkAny/normal-4           16595194                71.02 ns/op           24 B/op          1 allocs/op
BenchmarkAny/normal-8           16130025                72.98 ns/op           24 B/op          1 allocs/op
BenchmarkAny/normal_with_logger                  9092430               130.0 ns/op            88 B/op          2 allocs/op
BenchmarkAny/normal_with_logger-2               10157986               122.6 ns/op            88 B/op          2 allocs/op
BenchmarkAny/normal_with_logger-4                9523335               123.1 ns/op            88 B/op          2 allocs/op
BenchmarkAny/normal_with_logger-8                9183698               128.2 ns/op            88 B/op          2 allocs/op
BenchmarkAny/normal_new_goroutine                1250547               883.7 ns/op            73 B/op          2 allocs/op
BenchmarkAny/normal_new_goroutine-2              2108619               659.3 ns/op            72 B/op          2 allocs/op
BenchmarkAny/normal_new_goroutine-4              2919674               407.9 ns/op            72 B/op          2 allocs/op
BenchmarkAny/normal_new_goroutine-8              4778667               320.5 ns/op            72 B/op          2 allocs/op
PASS
ok      go.uber.org/zap 18.898s