Sync `let-go` upstream for embed surface improvements and remove workarounds

[jasalt]

• api: no supported way to detect incomplete input — embedder REPLs string-match error text nooga/let-go#587
• nrepl: server can't be embedded — requires a *compiler.Context that api never exposes nooga/let-go#586
• api: Run silently drops every form after the first — no multi-form evaluation path for embedders nooga/let-go#585