Redesign visit

[timholy]

This extensively reworks visit and visit_backedges:

• it requires operation to return a Bool to control descent
• it extends the "visit-once" logic to all objects
• it extends printing to all objects but turns it off by default
• it greatly improves performance with @nospecialize annotations
• it separates user-interface from internals (visit vs _visit)
• it supports more types (MethodTable, CodeInstance)
• it orthogonalizes visit and visit_backedges, allowing the latter to handle everything except the backedges themselves.

Closes #2

[codecov[bot]]

Codecov Report

Merging #3 into master will decrease coverage by 1.94%. The diff coverage is 93.68%.

@@            Coverage Diff             @@
##           master       #3      +/-   ##
==========================================
- Coverage   96.21%   94.26%   -1.95%     
==========================================
  Files           3        3              
  Lines         132      157      +25     
==========================================
+ Hits          127      148      +21     
- Misses          5        9       +4     

Impacted Files	Coverage Δ
src/visit.jl	91.95% <92.00%> (-6.56%)	:arrow_down:
src/MethodAnalysis.jl	93.33% <100.00%> (+0.47%)	:arrow_up:
src/backedges.jl	100.00% <100.00%> (+5.40%)	:arrow_up:

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