Ergonomic language features and lib cleanup

+ let bindings
+ where bindings
+ do notation

I explored enough of the alternative language design space and decided
that we should commit fully to Lambda style. That means no more highly
tacit/concatenative point-free/partial programs as default. We'll keep
taking advantage of those capabilities when it makes sense, but the
library will continue to see massive overhauls.

src/Eval.hs

@@ -9,6 +9,7 @@ import Data.List (partition, (\\), elemIndex, foldl')
 import Data.Map  ()
 import Data.Set  (Set)
 import Database.SQLite.Simple
+import Debug.Trace (trace)
 
 import qualified Data.Foldable as F ()
 import qualified Data.Map as Map