Rationalise Value to string methods

[jcamiel]

hurl::runner::value::Value implements many methods to be converted as a String. It's not clear which one we should call in which context.
At least, these methods and their usage should be documented in hurl::runner::value.rs.

• hurl/runner/value.rs:

impl fmt::Display for Value {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

impl Value {
    pub fn render(&self) -> Option<String> {

• hurl/runner/predicates.rs:

impl Value {
    pub fn display(&self) -> String {

Maybe also ?:

impl Value {
   pub(crate) fn format(&self) -> String {

This code "renders" a value to a string...

let value = Value::String("foo".to_string())
value.to_string();
value.render();
value.display();
value.format();