revamp MazeUndirected & MazeDirected

[Sid-Bhatia-0]

1. Remove old MazeUndirected and MazeDirected.
2. Add new MazeUndirectedModule.MazeUndirected and MazeDirectedModule.MazeDirected with improved implementation.
3. Update tests.
4. Update README.

Here is how the new environments look:

MazeUndirectedModule.MazeUndirected:

MazeDirectedModule.MazeDirected:

[codecov-commenter]

Codecov Report

Merging #156 (5cd9017) into master (010452d) will decrease coverage by 1.29%.
The diff coverage is 81.39%.

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
- Coverage   71.80%   70.51%   -1.30%     
==========================================
  Files          26       27       +1     
  Lines        2472     2479       +7     
==========================================
- Hits         1775     1748      -27     
- Misses        697      731      +34     

Impacted Files	Coverage Δ
src/envs/envs.jl	69.69% <ø> (ø)
src/envs/maze_directed.jl	71.42% <71.42%> (ø)
src/envs/maze_undirected.jl	86.00% <86.00%> (ø)
src/rlbase.jl	83.82% <87.50%> (+1.13%)	⬆️
src/envs/snake.jl	80.48% <0.00%> (-14.64%)	⬇️

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