Make new a const fn

[Swatinem]

This removes the initial capacity of 2 and rather makes new a const fn.

Fixes #49

[Amanieu]

LGTM once CI is fixed by #69.

[Swatinem]

The CI failure here looks like exactly what I had locally, and which is fixed by #68

[Swatinem]

and now that I think about it, I believe removing the default capacity of 2 is what exposed this problem, as likely the very first bucket is never created, and thus the broken code would early-return in that case.

[james7132]

This bumped the MSRV to 1.61, which should probably be reflected in the Cargo.toml.

  error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
     --> /home/runner/.cargo/registry/src/suiyiyu.us.kg-1ecc6299db9ec823/thread_local-1.1.8/src/lib.rs:154:6
      |
  154 | impl<T: Send> ThreadLocal<T> {
      |      ^
  155 |     /// Creates a new empty `ThreadLocal`.
  156 |     pub const fn new() -> ThreadLocal<T> {
      |     ------------------------------------ function declared as const here
      |
      = note: see issue #93706 <https://github.com/rust-lang/rust/issues/93706> for more information

[Amanieu]

Done