Add an Environment type to represent environment variables

[jakepetroules]

This starts with a slightly modified version of the implementation from SwiftPM. This is not yet adopted anywhere, it's just adding the initial implementation.

Closes #186

[jakepetroules]

@swift-ci test

[jakepetroules]

Ignore the "diffs" in this file. This PR is entirely additions; the original Environment.swift was renamed to EnvironmentHelpers.swift.

[jakepetroules]

@swift-ci test windows

[jakepetroules]

@swift-ci test

[jakepetroules]

@swift-ci test windows

[dschaefer2]

If you want to apply it somewhere, TouchTool could use some love.

            guard let commandShellPath = getEnvironmentVariable("ComSpec") else {

[jakepetroules]

@swift-ci test

[jakepetroules]

@swift-ci test

[jakepetroules]

@swift-ci test

[jakepetroules]

If you want to apply it somewhere, TouchTool could use some love.

            guard let commandShellPath = getEnvironmentVariable("ComSpec") else {

Coming in a follow-on PR :)

[weliveindetail]

@jakepetroules I am afraid your last patch causes a compile failure on Windows:

C:\Users\swift-ci\jenkins\workspace\swift-main-windows-toolchain-arm64\swift-build\Sources\SWBUtil\Environment.swift:16:19: error: cannot find type 'EnvironmentKey' in scope
 14 | 
 15 | public struct Environment {
 16 |     var storage: [EnvironmentKey: String]
    |                   `- error: cannot find type 'EnvironmentKey' in scope
 17 | }
 18 | 

Attaching the full lo here: swift-main-windows-toolchain-arm64-1060.log

As a result the nightly builds fail and PR testing is stuck. I will prepare a revert. Please let me know if you have a better proposal.