Require async feature for doctests and mosts tests

[jayvdb]

Closes #292

[jayvdb]

The alternative is to add the following

diff --git a/butane/Cargo.toml b/butane/Cargo.toml
index 6487f64..4184a54 100644
--- a/butane/Cargo.toml
+++ b/butane/Cargo.toml
@@ -36,6 +36,7 @@ r2d2 = { optional = true, workspace = true }
 deadpool = { optional = true, workspace = true }

 [dev-dependencies]
+butane = { workspace = true, features = ["async"] }
 butane_test_helper = { workspace = true }
 butane_test_macros = { workspace = true }
 cfg-if = { workspace = true }

happy to do that, if it is preferred.

[Electron100]

I'm merging this as a short-term measure but I'd like to leave the issue open because something seems fishy here. The way the test and product macros are written, they should skip generating or requiring any of the async stuff if async isn't enabled. So I worry this could be masking a real bug. Or at the very least I'd like to better understand and document what's going on. I'll try to take a closer look tomorrow.