fix slice assignment for strings

[gluax]

Fixes #1025.
It also removes valid programming checks for phases and only checks the AST.

[acoglio]

Looks good.

(But it appears that a required CI test did not pass.)

[gluax]

@acoglio I'm looking into it, all of them pass locally for me so I'm trying to figure out what's up.

[codecov]

Codecov Report

Merging #1027 (2be23c4) into master (316e781) will increase coverage by 0.40%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #1027      +/-   ##
==========================================
+ Coverage   75.35%   75.75%   +0.40%     
==========================================
  Files         438      438              
  Lines       17586    17601      +15     
==========================================
+ Hits        13252    13334      +82     
+ Misses       4334     4267      -67     

Impacted Files	Coverage Δ
compiler/src/compiler.rs	90.62% <ø> (ø)
compiler/src/console/assert.rs	81.81% <ø> (ø)
compiler/src/console/console.rs	100.00% <ø> (ø)
compiler/src/expression/array/access.rs	50.41% <ø> (+28.09%)	⬆️
compiler/src/expression/conditional/conditional.rs	71.42% <ø> (ø)
compiler/src/expression/expression.rs	88.09% <ø> (ø)
compiler/src/expression/logical/and.rs	90.00% <ø> (ø)
compiler/src/expression/logical/or.rs	90.00% <ø> (ø)
compiler/src/expression/relational/eq.rs	66.66% <ø> (ø)
compiler/src/expression/relational/ge.rs	66.66% <ø> (ø)
... and 40 more

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