[Heap] Final(?) Heap API adjustments for 1.1

[lorentey]

With thanks to @JadenGeller, land the following minor API improvements in Heap:

• Mark removeMin() and removeMax() as @discardableResult, emulating removeFirst()/removeLast() in the stdlib.
• Add a reserveCapacity() implementation that simply forwards to the similar method on the heap's array storage.
• Also add an init(minimumCapacity:) initializer that reserves capacity during heap creation. (This is generally a better idea than calling reserveCapacity on an already existing heap -- as it avoids getting called in a loop.)

Checklist

• [X] I've read the Contribution Guidelines
• [X] My contributions are licensed under the Swift license.
• [X] I've followed the coding style of the rest of the project.
• [ ] I've added tests covering all new code paths my change adds to the project (if appropriate).
• [X] I've added benchmarks covering new functionality (if appropriate).
• [X] I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• [ ] I've updated the documentation if necessary.

[lorentey]

(The macOS tests are expected to fail due to a CI issue. It does not indicate a problem with these changes.)

@swift-ci test

[lorentey]

D'oh, we'll need to update the DocC TOC too