Disallow modification of env of a struct or an enum

[yutannihilation]

Close #261

library(savvyExamples)

# struct
Person$new_method <- function() NULL
#> Error: Person cannot be modified

p <- Person$new()
p$.ptr <- NULL
#> Error: Person cannot be modified

# enum
FooEnum$A <- NULL
#> Error: FooEnum cannot be modified

FooEnum$C <- "C"
#> Error: FooEnum cannot be modified

a <- FooEnum$A
a$.ptr <- NULL
#> Error: FooEnum cannot be modified

^{Created on 2024-06-09 with reprex v2.1.0}