Fix baka let and add some improvement

[HoshinoTented]

Goals:

• Nested interface for nested structure (copied from Let Term #979)
• Better prettier for lambda
• Fixes baka let which has redundant lambda

TODOs:

• Better prettier for Pi (if Goal#2 is good) Pi is special

[codecov]

Codecov Report

Merging #982 (98a9915) into main (7fb9a1b) will decrease coverage by 0.40%.
The diff coverage is 81.81%.

@@             Coverage Diff              @@
##               main     #982      +/-   ##
============================================
- Coverage     81.98%   81.58%   -0.40%     
+ Complexity     3475     3444      -31     
============================================
  Files           280      281       +1     
  Lines         10509    10526      +17     
  Branches       1271     1271              
============================================
- Hits           8616     8588      -28     
- Misses         1176     1211      +35     
- Partials        717      727      +10     

Impacted Files	Coverage Δ
base/src/main/java/org/aya/concrete/Expr.java	85.63% <50.00%> (-0.42%)	⬇️
...c/main/java/org/aya/prettier/ConcretePrettier.java	82.46% <61.53%> (-0.65%)	⬇️
base/src/main/java/org/aya/generic/Nested.java	100.00% <100.00%> (ø)
base/src/main/java/org/aya/tyck/ExprTycker.java	84.00% <100.00%> (-0.15%)	⬇️
...c/main/java/org/aya/tyck/tycker/LetListTycker.java	100.00% <100.00%> (ø)
.../src/main/java/org/aya/tyck/tycker/MockTycker.java	100.00% <100.00%> (ø)

... and 10 files with indirect coverage changes

[HoshinoTented]

bors r+

[bors]

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