Improve performance of `HTTPHeaders.subscript(canonicalForm:)` (#1952)
Improve performance of `HTTPHeaders.subscript(canonicalForm:)` (#1952)

Motivation:

When getting the canonical form of header values any ascii whitespace
is stripped from the values. The current implementation does this by
doing equality checks on `Character` which is quite expensive.

Modifications:

- Update the `Substring.trimWhitespace()` function to trim on a UTF8
  view

Result:

Retrieving the canonical form of header values is cheaper, significantly
so when values contain whitespace.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio