Disallow dots in identifiers (#1267)

Disallow dots in identifiers (#1267)

* Disallow leading dot in identifiers

This is to avoid `alr printenv` printing invalid
environment variable names, i.e. with leading dot.

* Fix typo in run_alr error message

* Simplify crate name validation test code

* Use two letters for min crate name length verification

* Test init with valid crate name

* Add more test cases for bad index name

* Use constant instead of dot literal

* Disallow dots in identifiers

Dots in identifiers are allowed by mistake.
This commit forbids them completely, instead
of just disallowing dots as the first character
in crate name.