FR: using memory mapped buffers for file writes in Crashlytics

### Feature proposal

* Firebase Component: Crashlytics

This is not so much a feature request as an RFC, since I'm happy to open a PR with this change and I just want to get consensus on it first. In `FIRCLSFileWriteToFileDescriptorOrBuffer` we see that a hard decision is made between durability and speed by having the option to buffer. We also see a `write` syscall when the buffering is done, which is of course expensive. If we use mmap instead, we can do fast, unbuffered writes directly to kernel memory. We can have our cake and eat it too.