Add constant folding and lookup function to transform plugin

[timggggggg]

Description

• add lookup(value, table, default:) — translates a value through an
  object table; keys match by string form, so 0 and "0" are the same key.
  A miss returns the value unchanged unless default: is given.
• fold constant call arguments and assignment values into core.ConstExpr
  during the validation walk, so a table literal is built once at startup
  instead of per event (70 entries: 7347 -> 1248 ns/op, 88 -> 14 allocs).
  An expression that fails to evaluate is never folded and keeps failing
  at runtime.
• fix equality against event fields: JSONNodeValue compared String()
  forms, so .obj == {a: 1} could never match. == and != now resolve both
  operands like every other operator. Equality is now type-aware:
  .num == "1" is false where it used to be true.
• sort object keys in DumpAST

Fixes #999