update bunch of miscellaneous things

[Sid-Bhatia-0]

1. Update RLBase.action_space to return Base.OneTo(NUM_ACTIONS) for all environments. All environments have discrete action spaces and this is a consistent way to refer to them.
2. Rename AbstractGridWorldGame to AbstractGridWorld.
3. Rename get_tile_map_height to get_height and get_tile_map_width to get_width.
4. Rename get_tile_pretty_repr and get_tile_map_pretty_repr to get_pretty_tile_map.
5. Rename get_sub_tile_pretty_repr and get_sub_tile_map_pretty_repr to get_pretty_sub_tile_map.
6. Update the miscellaneous section for all environments. Use CartesianIndex as argument instead of passing i and j separately to get_pretty_tile_map. Add method get_object_names and print it along with action_names in the show method. Add sub_tile_map view to almost all environments.
7. Fix benchmarks.jl. Remove RLBaseEnvModule.
8. Update README.

[codecov-commenter]

Codecov Report

Merging #174 (cd7194f) into master (f05079c) will decrease coverage by 9.40%. The diff coverage is 3.88%.

@@            Coverage Diff             @@
##           master     #174      +/-   ##
==========================================
- Coverage   72.40%   62.99%   -9.41%     
==========================================
  Files          27       27              
  Lines        2290     2654     +364     
==========================================
+ Hits         1658     1672      +14     
- Misses        632      982     +350     

Impacted Files	Coverage Δ
src/abstract_grid_world.jl	11.92% <0.00%> (-0.12%)	:arrow_down:
src/play.jl	0.00% <0.00%> (ø)
src/rlbase.jl	20.00% <0.00%> (ø)
src/envs/go_to_target_directed.jl	56.17% <2.94%> (-18.45%)	:arrow_down:
src/envs/door_key_directed.jl	56.81% <3.03%> (-17.81%)	:arrow_down:
src/envs/dynamic_obstacles_directed.jl	57.95% <3.12%> (-14.11%)	:arrow_down:
src/envs/grid_rooms_directed.jl	55.95% <3.12%> (-17.49%)	:arrow_down:
src/envs/maze_directed.jl	55.95% <3.12%> (-17.49%)	:arrow_down:
src/envs/sokoban/sokoban_directed.jl	57.60% <3.12%> (-17.04%)	:arrow_down:
src/envs/collect_gems_directed.jl	57.14% <3.22%> (-16.71%)	:arrow_down:
... and 18 more

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