Optimize memory usage for 2 core functions

[gbuisson]

During our load tests we noticed heavy memory usage for both is-sub-list? and repr-is-subscope?.
while their implementation is perfectly readable and favors maintenability, we can slightly tweak them to improve performance, this is an attempt at it, keeping a conservative approach while slightly tunning the code to reduce the memory footprint.

[yogsototh]

I checked with criterium, this is slightly faster, not sure however how to check if this will use more or less memory.

Edit: I could test:

Initial used heap: 61.1 MiB (1.5%)
│ 
│ (scopula.core-bench/t <184 B>) | Heap: +504 B (+0.0%)
│ │ 
│ │ (scopula.core/is-sub-list? <416 B> <416 B>) | Heap: +21.8 KiB (+0.0%)
│ │ │ 
│ │ └─→ <16 B> | Heap: +35.1 KiB (+0.0%)
│ │ 
│ └─→ <16 B> | Heap: +34.8 KiB (+0.0%)
Final used heap: +34.7 KiB (+0.0%)

The new version:

Initial used heap: 61.2 MiB (1.5%)
│ 
│ (scopula.core-bench/t <192 B>) | Heap: +456 B (+0.0%)
│ │ 
│ │ (scopula.core-bench/is-sub-list-2 <416 B> <416 B>) | Heap: +3.5 KiB (+0.0%)
│ │ │ 
│ │ └─→ <16 B> | Heap: +9.4 KiB (+0.0%)
│ │ 
│ └─→ <16 B> | Heap: +9.8 KiB (+0.0%)
Final used heap: +9.7 KiB (+0.0%)