Update toml from 0.5 to 0.8

[dtolnay]

The toml crate has a much improved parser based on toml_edit, which produces more accurate error messages.

Before:

[ERROR rust_team] failed to parse teams/libs.toml
    
    Caused by:
        invalid type: map, expected a sequence for key `github` at line 53 column 1

After:

[ERROR rust_team] failed to parse teams/libs.toml
    
    Caused by:
        TOML parse error at line 20, column 1
           |
        20 | [github]
           | ^^^^^^^^
        invalid type: map, expected a sequence