Normalize source directory structure (lib vs src)

[paolino]

Some libraries use src/ as source directory, others use lib/. This inconsistency makes tooling harder.

Libraries using lib/:

• application-extras
• application-tls
• coin-selection
• balance-tx
• primitive
• address-derivation-discovery

Libraries using src/:

• delta-types
• delta-store
• delta-chain
• delta-table
• text-class
• numeric
• test-utils
• etc.

Should normalize to one convention (suggest src/ as it's more standard).