chore: improve users util

[harrysolovay]

Fixes #760

Enables one to use createUsers (renamed from users) without specifying a count, which will cause it to default to a record (not a list) of 26 named Sr25519s.

const { alexa, billy, carol, david } = await createUsers()

Count specificity still works as is did before.

const [a, b, c, d] = await createUsers(4)

[harrysolovay]

Will this be confusing? / will devs think that accessing a named user from multiple users() calls will yield the same Sr25519? If so, perhaps we should rename the util to nextUsers or something that indicates the creation of a new set.

EDIT

Yea, this was going to be confusing. Hence we've renamed users to createUsers.