Compile checks for constants do not work

[egroge]

Semantic analysis does not successfully test for reassignments to constants. At the moment, some limited checking exists. For example, the following contract would not compile:

contract A {
  let a: Int = 0
}

A :: (any) {
  public init() {
    self.a = 5 // Error
  }
}

However, note that if self.a = 5 were instead a = 5, then the error would not be caught.
This kind of check also works for local variables, such as the following example, were it part of a top level declaration:

public func foo() {
  let my_const: Int = 0
  my_const = 1 // Error
}

However, there are cases where the checker allows reassignments through when it should not, such as:

contract A {
  let a: Int
}

A :: (any) {
  public init() {
    a = 0
  }

  public func foo() mutates(a) {
    a = 1 // or self.a = 1
  }
}