Avoid unnecessary heap allocation when parsing frame headers. (#107)
Avoid unnecessary heap allocation when parsing frame headers. (#107)

Motivation:

As noted in #105, I spotted that we were heap-allocating an array
when parsing the frame header. That heap allocation isn't necessary, we
can just write directly into an Int, which should save us some heap spray.

Modifications:

- Rewrote code to avoid heap allocating array and instead to use Int.
- Elided some bounds checks by using an unsafe pointer, since I was
    already here and it saved about 1% performance in a hot loop.

Result:

Fewer heap allocations on the hot code path.
apple/swift-nio-http2,apple/swift-nio-http2,apple/swift-nio-http2,apple/swift-nio-http2