Replace: Fix instantiations with unions

[som-sm]

Fixes behaviour of Replace when instantiated with unions.

1. When Search is a union

   type Current = Replace<'bar', 'a' | 'b', 'z'>;
   //   ^? type Current = "zr" | "zar" | "bzr" | "bzar"
   
   type Updated = Replace<'bar', 'a' | 'b', 'z'>;
   //   ^? type Updated = "bzr" | "zar"

   type Current = Replace<'barbar', 'a' | 'b', 'z', {all: true}>;
   //   ^? type Current = "zrbzr" | "zrbzzr" | "zrzr" | "zrzzr" | "zzzr" | "zzrbzr" | "zzrbzzr" | "zzrzr" | "zzrzzr" | "zarzzr" | "zarzrbzr" | "zarzrbzzr" | "zarzrzr" | "zarzrzzr" | "bzrbzr" | "bzrbzzr" | ... 11 more ... | "bzarzrzzr"
   
   type Updated = Replace<'barbar', 'a' | 'b', 'z', {all: true}>;
   //   ^? type Updated = "bzrbzr" | "zarzar"

2. When Replacement is a union

   type Current = Replace<'barbar', 'a', 'x' | 'z', {all: true}>;
   //   ^? type Current = "bxrbxr" | "bxrbzr" | "bzrbxr" | "bzrbzr"
   
   type Updated = Replace<'barbar', 'a', 'x' | 'z', {all: true}>;
   //   ^? type Updated = "bxrbxr" | "bzrbzr"