Make FruitHandle and MultiFruit public

[ppodolsky]

Hi! I've been coding dynamic Collector creation in my project and figured out that it was impossible to store FruitHandle inside struct or Vec because of (in)visibility of FruitHandle and MultiFruit. Is it ok to make them visible?

[codecov-commenter]

Codecov Report

Merging #1360 (fdea97e) into main (944302a) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1360   +/-   ##
=======================================
  Coverage   94.28%   94.29%           
=======================================
  Files         234      234           
  Lines       42528    42528           
=======================================
+ Hits        40099    40102    +3     
+ Misses       2429     2426    -3     

Impacted Files	Coverage Δ
src/collector/mod.rs	48.21% <ø> (ø)
src/collector/multi_collector.rs	99.30% <ø> (ø)
src/fastfield/serializer/mod.rs	92.75% <0.00%> (+1.44%)	⬆️
src/directory/directory.rs	95.58% <0.00%> (+1.47%)	⬆️

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