feat: scaffold crate structure and implement core macros

Add the project skeleton, LICENSE files, README, Makefile, doc
examples, and the initial implementation of `#[func]`, `invoke!`,
and `select!` procedural macros.

1 files changed, 22 insertions, 0 deletions

diff --git a/doc/select.md b/doc/select.md
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+Procedural macro that chooses between two expressions based on whether the
+async feature is enabled, producing a value.
+
+## Explicit mode
+
+Each arm is labelled with a feature name. When the named feature is enabled
+that arm is selected; otherwise the other arm is used. Since only one feature
+is ever active at a time, the arm labelled `"sync"` effectively acts as an else
+branch.
+
+The `!` token inverts the sense — it selects the arm when the corresponding
+feature is **not** enabled.
+
+## Implicit mode
+
+Feature names are omitted; the macro inspects the token stream to decide which
+expression is async. Whichever arm contains a `.await` call is treated as the
+async branch.
+
+When neither arm contains `.await`, the first expression is assigned to the
+async branch and the second to sync. The two branches trade places during
+expansion so the correct one is active in each mode.