add support for memoryview objects

[jakkdl]

Fixes boto/boto3#3423

I know it's been labeled needs-discussion, but I wanted to check if it really was as easy as just modifying a couple isinstance checks, and after trying it and seeing it really was just that I thought I might as well write up a PR for it.

I've been working on piskvorky/smart_open#380 and it would save significant amounts of memory if it was possible to directly pass memoryview objects to botocore.

I opted to modify all cases where I found isinstance.*bytearray in the code, even though boto/boto3#3423 only directly needs the modifications in validate.py and utils.py afaik. But I see no reason not to accept it in all those places.

I also noticed there was no test for bytearray + httpchecksum, and the error message for type validation didn't include str despite the isinstance line above accepting it, so quickly fixed those as well.