Better flow-sensitive typing

[apiology]

Cases now at least partially handled:

• if foo.nil?
• if foo
• unless foo.nil? || foo
• if unrelated && foo
• break if foo.nil?
• return foo unless !foo
• return if foo.nil?
• foo = nilable || not_nilable
• foo unless foo.nil? || foo
• foo if unrelated && foo
• while foo
• foo unless unrelated || repr.nil?
• break unless is_a?
• case ... when ... return if ...

Includes for functionality:

• https://github.com/castwide/solargraph/pull/1119

Includes for convenience:

• https://github.com/castwide/solargraph/pull/1102

Includes for better testing:

• https://github.com/castwide/solargraph/pull/1087

Includes for buildability:

• https://github.com/castwide/solargraph/pull/1120

[apiology]

I'm pulling good bits out of https://github.com/apiology/solargraph/pull/12 - feel free to merge this PR at any point that it's green, I'll probably add more stuff over time while I improve the other PR.