SwiftNIO 2.8.0

SemVer Minor

• Allow promises to be completed with a Result<Value, Error> (#1124, patch credit to @glbrntt)
• ChannelPipeline: fail double removal of handler (#1133, reported by @vlm)

SemVer Patch

• fix NIODeadline/TimeAmount maths (#1136, reported by @mariosangiorgio)
• HTTPServerUpgradeHandler: Tolerate futures from other ELs (#1134, reported by @vlm)
• Lock: guarantee crash instead of undefined behaviour (#1131, reported by @kevints)
• Avoid having cascade be title of all methods in jazzy docs (#1129, patch credit to @ktoso)
• Fix doc typo: before possible -> are possible (#1122, patch credit to @tamc)