Default type parameter of WalkContext and AbstractWalker to void

[andy-hanson]

PR checklist

• Addresses an existing issue: #0000
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Overview of change:

Most rules take no options, so it's simpler to not require a type parameter.

CHANGELOG.md entry:

[api] WalkContext and AbstractWalker type parameter defaults to void

[adidahiya]

Doesn't this force custom rules to now use TS 2.3? I consider that a breaking change

[giladgray]

at this point i think a minimum TS dep of 2.3 is fine.

@andy-hanson can you please update this PR? or close it.

[andy-hanson]

Had to commit with git commit --no-verify or else there was a huge number of changes.