Emit #[allow(non_snake_case)] for constructor identifier

[maxdeviant]

This PR adds #[allow(non_snake_case)] to the emitted constructor identifier to prevent #[warn(non_snake_case)] from complaining in some situations.

The suffix used when generating the constructor identifier already triggers this warning, so regardless of the input identifier the generated identifier is already in violation of this lint:

λ cargo test
   Compiling ctor v0.2.5 (/Users/maxdeviant/projects/rust-ctor/ctor)
warning: function `foo___rust_ctor___ctor` should have a snake case name
  --> ctor/src/example.rs:10:4
   |
10 | fn foo___rust_ctor___ctor() {}
   |    ^^^^^^^^^^^^^^^^^^^^^^ help: convert the identifier to snake case: `foo_rust_ctor_ctor`
   |
   = note: `#[warn(non_snake_case)]` on by default

warning: `ctor` (example "example") generated 1 warning

[TennyZhuang]

@mmastrac Can you release a new minor or patch version for the commit? Thanks! It's important for us to eliminate the warnings in our codebase.

[mmastrac]

Released in ctor v0.2.6