Feature names beginning with numbers break builds

[tazjin]

Issue description

Some Rust crates (most prominently Diesel) have features with
names that start with numbers.

In the current implementation of Carnix, these generate unquoted
attribute set keys which end up breaking the Nix parser.

Example from the Cargo.nix of a project depending on Diesel:

  diesel_1_2_2_features = f: updateFeatures f (rec {
    bitflags_1_0_2.default = true;
    byteorder_1_2_2.default = true;

                 ----- This breaks Nix
                 v
    diesel_1_2_2.128-column-tables =
      (f.diesel_1_2_2.128-column-tables or false) ||
      (f.diesel_1_2_2.x128-column-tables or false) ||
      (diesel_1_2_2.x128-column-tables or false);

                 ----- Same here
                 v
    diesel_1_2_2.32-column-tables = ....;

Leads to errors like this:

error: syntax error, unexpected FLOAT, expecting '.' or '=', at /home/vincent/projects/REDACTED/Cargo.nix:2358:17

An easy fix would be to simply quote all feature names.

[tazjin]

This issue is fixed on the devel branch.