fixes #21393 and misc style changes (#21419)

* fixes #21393 and misc style changes

* progress

---------

Co-authored-by: ringabout <43030857+ringabout@users.noreply.github.com>

lib/system/seqs_v2.nim

@@ -83,7 +83,8 @@ proc shrink*[T](x: var seq[T]; newLen: Natural) {.tags: [], raises: [].} =
       for i in countdown(x.len - 1, newLen):
         reset x[i]
     # XXX This is wrong for const seqs that were moved into 'x'!
-    cast[ptr NimSeqV2[T]](addr x).len = newLen
+    {.noSideEffect.}:
+      cast[ptr NimSeqV2[T]](addr x).len = newLen
 
 proc grow*[T](x: var seq[T]; newLen: Natural; value: T) =
   let oldLen = x.len

lib/system/strmantle.nim

@@ -33,7 +33,7 @@ proc eqStrings(a, b: string): bool {.inline, compilerproc.} =
 proc hashString(s: string): int {.compilerproc.} =
   # the compiler needs exactly the same hash function!
   # this used to be used for efficient generation of string case statements
-  var h : uint = 0
+  var h = 0'u
   for i in 0..len(s)-1:
     h = h + uint(s[i])
     h = h + h shl 10