Improve method.isConst() checks in InvokeMethodVirtualNode

- Check `method.isConst()` every time a method is resolved
  instead of once per node instance (`isConstChecked`).
  Given that `needsConst` only happens in very specific circumstances,
  I'm not entirely sure if every resolved method needs to be checked.
  However, it's a cleaner solution in any case, and `method.isConst()`
  is a fast check that also never happens on the `evalCached` fast path.
- Do not check for const-ness of `FunctionN.apply` methods.
  This check seems unnecessary and would always fail if triggered.
  (According to `base.pkl`, none of the `FunctionN.apply` methods is const.)
- Remove unnecessary Truffle boundaries for modifier checks,
  which are just bitwise operations.
- Improve const/import docs.

docs/modules/language-reference/pages/index.adoc

@@ -989,8 +989,8 @@ Because a `local` property is added to the lexical scope, but not (observably) t
 ====
 
 An _import clause_ defines a local property in the containing module.
-This means `import "someModule.pkl"` is equivalent to `local someModule = import("someModule.pkl")`.
-Also, `import "someModule.pkl" as otherName` is equivalent to `local otherName = import("someModule.pkl")`.
+This means `import "someModule.pkl"` is effectively `const local someModule = import("someModule.pkl")`.
+Also, `import "someModule.pkl" as otherName` is effectively `const local otherName = import("someModule.pkl")`.
 ====
 
 [[fixed-properties]]