Remove type parameters for SharedStatsStep and PooledStatsProcedure

[junyuan-chen]

pool seems to be faster now.

Additionally, adding @inline helps the performance of coef and vcov when getting indices based on the rows in treatinds.

[codecov]

Codecov Report

Merging #9 (f88411c) into master (11ff4aa) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master       #9   +/-   ##
=======================================
  Coverage   99.60%   99.60%           
=======================================
  Files           8        8           
  Lines         503      506    +3     
=======================================
+ Hits          501      504    +3     
  Misses          2        2           

Impacted Files	Coverage Δ
src/StatsProcedures.jl	99.19% <100.00%> (+<0.01%)	⬆️
src/did.jl	100.00% <100.00%> (ø)

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