aws/corehandlers: speed up validation of []byte

[rhysh]

Request validation logic ensures that struct fields have appropriate values.
When validating slices and maps, the goal is to find any structs they may
contain, since only struct fields have validation rules. Slices of uint8,
aka []byte, do not lead to structs and so cannot cause validation to fail.
Speed up validation of this (unnamed) type.

This is particularly important for APIs such as Kinesis which can contain
byte slices for megabytes of data - each byte does not need to be validated
individually.

The below output of golang.org/x/tools/cmd/benchcmp is for a Kinesis
PutRecords request including 100 Records of 10kB each.

benchmark old ns/op new ns/op delta
BenchmarkValidateAny-8 310052182 216116 -99.93%

benchmark old allocs new allocs delta
BenchmarkValidateAny-8 3002424 1909 -99.94%

benchmark old bytes new bytes delta
BenchmarkValidateAny-8 48079408 19274 -99.96%

[jasdel]

Looks really good, thanks for taking the time to put together this PR. The only comment I have is a minor tweak to the method used to determine if the value is a []byte

// precached byte slice reflect type for comparison
var byteSliceType = reflect.TypeOf([]byte(nil))

// ...
func (v *validator) validateAny(value reflect.Value, path string) {
// ...
    case reflect.Slice:
        if value.Type() == byteSliceType {
            // Skip byte slice types since there is no validation on content
            // inside the byte slice.
            return
        }
// ...

This method I think is a little easier to read, and allocates the same memory. Over several benchmark runs it seems to be about 4000 ns/op faster, but that could easily be just my system.

[rhysh]

Yes, that's a lot easier to read. Thanks!

[jasdel]

Looks good thanks for taking the time to create the PR. This will improve the resource usage of the SDK on any API operation with []byte as input parameters.