tutorial: clearer explanation of freezing.

- "Lending an immutable pointer" might be confusing. It was not discussed why borrowed pointers are immutable in the first place.
- Make it clear that the borrowed pointers are immutable even if the variable was declared with `mut`.
- Make it clear that we cannot even assign anything to the variable while its value is being borrowed.

tutorial: change "--" to an em-dash.

tutorial: change instances of "--" to em-dash.
auto merge of #12547 : jagtalon/rust/jag/rust/tutorial-freezing, r=pnkfelix

- "Lending an immutable pointer" might be confusing. It was not discussed why borrowed pointers are immutable in the first place.
- Make it clear that the borrowed pointers are immutable even if the variable was declared with `mut`.
- Make it clear that we cannot even assign anything to the variable while its value is being borrowed.

tutorial: change "--" to an em-dash.

tutorial: change instances of "--" to em-dash.
tutorial: Missing tildes around .notrust block

Adds a missing tilde to the end and the start of two .notrust blocks.
Rename a few typeck method-related structures to UpperCamelCase.
Replace callee_id with information stored in method_map.
auto merge of #12571 : eddyb/rust/kill-callee-id, r=nikomatsakis

Every method call and overloaded operator had a `callee_id` that was be used to store the method type and type substitutions, that information is now stored in the `method_map`, alongside the method's origin.
auto merge of #12570 : kyrias/rust/master, r=cmr

Adds a missing tilde to the end and the start of two .notrust blocks.