This pre-release adds support for Protocol 18.

horizonclient

The restriction that Fund can only be called on the DefaultTestNetClient has been removed. Any horizonclient.Client may now call Fund. Horizon instances not supporting funding will error with a resource not found error.
Change AccountRequest to accept Sponsor and LiquidityPool filters
Change EffectRequest, TransactionRequest, and OperationRequest to accept a ForLiquidityPool filter
Change TradeRequest to accept both a ForLiquidityPool filter or a TradeType filter
Add LiquidityPoolsRequest for getting details about liquidity pools, with an optional Reserves field to filter by pools' reserve asset(s).
Add LiquidityPoolRequest for getting details about a specific liquidity pool via the LiquidityPoolID filter.

GenericTransaction, Transaction, and FeeBumpTransaction now implement
encoding.TextMarshaler and encoding.TextUnmarshaler.
New asset structures that conform to the new ChangeTrust and New assets:
Support for the core liquidity pool XDR types: LiquidityPoolId, LiquidityPoolParameters, LiquidityPoolDeposit, and LiquidityPoolWithdraw.
Support for the new asset structures: ChangeTrustAsset and TrustLineAsset.

There's now a 5-minute grace period to transaction.ReadChallengeTx's minimum time bound constraint (#3824).
Assets can now be liquidity pool shares (AssetTypePoolShare).
All asset objects can now be converted to the new ChangeTrustAsset and TrustLineAsset objects.
Assets can now be compared in accordance with the protocol, see their respective LessThan() implementations.