Create cosmwasm-core

[webmaster128]

Following up on the discussion in #1956.

• Align version between cosmwasm-core and cosmwasm-std
• Add cosmwasm-core to table of crates in repo root README.md
• Move types
  • Binary
  • HexBinary
  • Addr
  • CanonicalAddr
  • Uint*/Int*
• Make base64/hex encoders/decoders public (see Implement From<&Vec> for Binary and HexBinary #2036 (review))?
• Add CI tests for cosmwasm-core, including no_std tests