Refactor/slots

[egasimus]

All internal changes that should not change the public API (much):

• override and Overridable as foundation for flexible parameter passing
• standardize string specifiers (type Into*)
• remove Artifact as separate class - only Source, Template and Contract remain
• baking TemplateSlot into Template and ContractSlot into Contract
• (?) deprecate Client in favor of Contract?
• add abstract classes for core workflow (Builder, Uploader) to @fadroma/client
• AgentContext + ChainContext -> ConnectContext
• use EnvConfig to parse configs from environment
• unifying logger/reporter utility objects
• etc...