Mutability and aliasing
=======================

Require `var` on both local variable/parameter, and on any fields,
in order to allow modification.

Disallow modification once local variable (or parameter) declared `var`
is either:
* Stored in a field that is neither `var` nor `aliased`.
* Passed to a `keep` parameter
* Passed to a self-`keep` method

This should be sufficient for preventing fields that are neither `var`
nor `aliased` from being aliased (and thus preventing them from being changed
behind ones back).

Keywords to use?
----------------

Mutability:

    `var`
    `mut`
    `modifiable`

Aliasing:

    `aliased`
    `link`
    `linked`
    `live`
    `nonfrozen`
    `othervar`
    `ref`
    `view`

Keep:

    `keep`

Embedding (preventing pointer indirection):

    `embed`