The current serde_derive's MSRV is 1.56, and both underscore consts and
ptr::addr_of! are always available.

Remove build script from serde_derive

…cess

They are used only in one place each, so for simplifying understanding it is better to inline them

Eliminate some duplicated code

    warning: this loop could be written as a `for` loop
        --> serde/src/private/de.rs:2905:9
         |
    2905 |         while let Some(item) = self.iter.next() {
         |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `for item in self.iter.by_ref()`
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#while_let_on_iterator
         = note: `#[warn(clippy::while_let_on_iterator)]` on by default