storage: add optional prefix iterator methods #1478
Merged
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Subspace key iterators are created with the iter_prefix() method on the
storage trait, which takes a string as a key prefix. Because the empty
string should be an invalid key for parsing purposes, other topics have
changed key parsing to error on the empty string. However, this leaves
us without a way to iterate over all subspace keys, previously achieved
with iter_prefix("").
To remedy this, iter_prefix has been extended into
iter_optional_prefix, taking an optional Key. Calls to iter_prefix("")
can be replaced with iter_all(), a new method; iter_prefix() still
exists with its original signature but will no longer accept empty
strings.