♻️ (minimal): Reimplementa join! y join_pair!

Usa `[&str]::concat()` de la biblioteca estándar y elimina la
dependencia de `concat-string`. Cada argumento se evalúa una sola vez y
no se consume.

Cargo.lock

@@ -396,12 +396,6 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
-[[package]]
-name = "concat-string"
-version = "1.0.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7439becb5fafc780b6f4de382b1a7a3e70234afe783854a4702ee8adbb838609"
-
 [[package]]
 name = "concurrent-queue"
 version = "2.5.0"
@@ -1892,7 +1886,6 @@ dependencies = [
 name = "pagetop-minimal"
 version = "0.1.0"
 dependencies = [
- "concat-string",
  "indoc",
  "pastey",
 ]