Add lengthable?

[primo-ppcg]

Related: #1279

• Add lengthable?

(lengthable? x)

Check if x is a bytes, indexed, or dictionary.

• Speed up keys, values, and pairs, while allowing non-lengthable types
• Allow non-lengthable types for reverse
• Minor speed improvement for reverse!
• Updates some docstrings missed in previous PRs

[sogaiu]

Various tests didn't turn up anything here 👍

Some manual trials:

$ ./build/janet
Janet 1.30.0-4cb1f616 linux/x64/gcc - '(doc)' for help

repl:1:> (lengthable? :a)
true

repl:2:> (lengthable? "a")
true

repl:3:> (lengthable? 1)
false

repl:4:> (lengthable? [:x :y])
true

repl:5:> (lengthable? (peg/compile '':a))
false

repl:6:> (lengthable? {:x 1})
true