[API] Use int32 instead of int8/16

[sotetsuk]

Found some buggy behaviors in int8 (e.g., failed to indexing via int8 var)

[codecov]

Codecov Report

Merging #1074 (37748df) into main (922a45e) will not change coverage.
The diff coverage is 99.52%.

@@           Coverage Diff           @@
##             main    #1074   +/-   ##
=======================================
  Coverage   88.03%   88.03%           
=======================================
  Files          54       54           
  Lines        5900     5900           
=======================================
  Hits         5194     5194           
  Misses        706      706           

Files	Coverage Δ
pgx/_src/api_test.py	98.83% <100.00%> (ø)
pgx/_src/chess_utils.py	100.00% <100.00%> (ø)
pgx/_src/gardner_chess_utils.py	100.00% <100.00%> (ø)
pgx/_src/shogi_utils.py	99.31% <100.00%> (ø)
pgx/animal_shogi.py	99.00% <100.00%> (ø)
pgx/backgammon.py	99.07% <100.00%> (ø)
pgx/chess.py	99.49% <100.00%> (ø)
pgx/connect_four.py	97.61% <100.00%> (ø)
pgx/experimental/bridge_bidding.py	0.00% <ø> (ø)
pgx/gardner_chess.py	99.30% <100.00%> (ø)
... and 10 more