Blocks in while-expr should enforce Unit type

[swernli]

In addition to fixing while-expr, this applies the same fix to for-loop, repeat-loop, and within-block, including unit tests for the desired type inference behavior.

Fixes #1251

[github-actions]

Benchmark for 39ac3da

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Test	Base	PR	%
Array append evaluation	339.2±18.09µs	352.0±3.02µs	+3.77%
Array literal evaluation	175.2±0.65µs	175.1±1.95µs	-0.06%
Array update evaluation	416.2±2.66µs	419.6±3.43µs	+0.82%
Core + Standard library compilation	22.5±0.48ms	24.4±1.22ms	+8.44%
Deutsch-Jozsa evaluation	5.0±0.07ms	5.2±0.13ms	+4.00%
Large file parity evaluation	34.7±0.23ms	34.6±0.67ms	-0.29%
Large input file compilation	14.1±0.26ms	14.5±0.46ms	+2.84%
Large input file compilation (interpreter)	54.3±1.85ms	56.6±1.51ms	+4.24%
Large nested iteration	33.2±0.36ms	33.4±0.28ms	+0.60%
Perform Runtime Capabilities Analysis (RCA) on Deutsch-Jozsa sample	1563.4±43.87µs	1594.1±92.95µs	+1.96%
Perform Runtime Capabilities Analysis (RCA) on large file sample	7.9±0.06ms	7.9±0.08ms	0.00%
Perform Runtime Capabilities Analysis (RCA) on teleport sample	1433.1±72.46µs	1452.6±74.18µs	+1.36%
Perform Runtime Capabilities Analysis (RCA) on the core and std libraries	27.8±0.28ms	28.2±0.29ms	+1.44%
Teleport evaluation	94.4±8.39µs	97.3±5.40µs	+3.07%