Cache symbol completion result

[tompng]

Make symbol completion faster

• Cache Symbol.all_symbols.sort and reuse it.
• Restrict the number of symbol candidates to reduce completion/rendering cost in Reline.
• Use bsearch to perform O(logN) search from cached sorted symbol list.

irb -f
irb(main):001> s='a';symbols = 100000.times.map{s=s.succ; [s.to_sym, ('a'+s).to_sym]};
irb(main):002> :aaabq
               :aaabk 
               :aaabl 
               :aaabm 
               :aaabn 
               :aaabo 
               :aaabp 
               :aaabq▄
               :azyd █
               :azye ▀
               :azyf  
               :azyg  
               :azyh  
               :azyi  
               :azyj  
               :azyk  

Completion candidates of symbol contains first 50 + last 50 symbols matched to the completion target.

[tompng]

I think it is relatively safe because it checks the value strictly, but it's a bit hacky...