Nix can perform static scope checking, but whenever code is inside
a `with` expression, the analysis breaks down, because it can't
know statically what's in the attribute set whose attributes were
brought into scope. In those cases, Nix has to assume that
everything works out.

Except it doesnt. Removing `with` from lib/ revealed an undefined
variable in an error message.

If that doesn't convince you that we're better off without `with`,
I can tell you that this PR results in a 3% evaluation performance
improvement because Nix can look up local variables by index.
This adds up with applications like the module system.

Furthermore, removing `with` makes the binding site of each
variable obvious, which helps with comprehension.

Unlike the other three is* functions in lib.trivial, it was only
available as lib.trivial.isFloat

This puts it among the trace* family of functions derived from it.

lib: Use Nix's static scope checking, fix error message, optimize