Empty R package with Rust implementation

Using in R:
```R
usethis::create_package(‘path/to/libkrigingtemplate’)
setwd(‘/path/to/libkrigingtemplate/‘)

cf https://extendr.github.io/rextendr/articles/package.html
```

.gitignore

@@ -0,0 +1,12 @@
+.idea
+/*.tar.gz
+/.Rproj.user
+/.Rhistory
+/.RData
+/.Ruserdata
+*.a
+*.o
+*.so
+/src/rust/vendor
+/src/rust/vendor.tar.xz
+/src/rust/**/target

DESCRIPTION

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+Package: libkrigingtemplate
+Title: What the Package Does (One Line, Title Case)
+Version: 0.0.0.9000
+Authors@R: 
+    person("First", "Last", , "first.last@example.com", role = c("aut", "cre"),
+           comment = c(ORCID = "YOUR-ORCID-ID"))
+Description: What the package does (one paragraph).
+License: `use_mit_license()`, `use_gpl3_license()` or friends to pick a
+    license
+Encoding: UTF-8
+Roxygen: list(markdown = TRUE)
+RoxygenNote: 7.2.1

NAMESPACE

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+# Generated by roxygen2: do not edit by hand
+

R/extendr-wrappers.R

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+#' @docType package
+#' @usage NULL
+#' @useDynLib libkrigingtemplate, .registration = TRUE
+NULL
+
+#' Return string `"Hello world!"` to R.
+#' @export
+hello_world <- function() .Call(wrap__hello_world)

src/.gitignore

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+*.o
+*.so
+*.dll
+target

src/Makevars

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+LIBDIR = ./rust/target/release
+STATLIB = $(LIBDIR)/liblibkrigingtemplate.a
+PKG_LIBS = -L$(LIBDIR) -llibkrigingtemplate
+
+all: C_clean
+
+$(SHLIB): $(STATLIB)
+
+$(STATLIB):
+	cargo build --lib --release --manifest-path=./rust/Cargo.toml
+
+C_clean:
+	rm -Rf $(SHLIB) $(STATLIB) $(OBJECTS)
+
+clean:
+	rm -Rf $(SHLIB) $(STATLIB) $(OBJECTS) rust/target

src/Makevars.win

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+TARGET = $(subst 64,x86_64,$(subst 32,i686,$(WIN)))-pc-windows-gnu
+LIBDIR = ./rust/target/$(TARGET)/release
+STATLIB = $(LIBDIR)/liblibkrigingtemplate.a
+PKG_LIBS = -L$(LIBDIR) -llibkrigingtemplate -lws2_32 -ladvapi32 -luserenv
+
+all: C_clean
+
+$(SHLIB): $(STATLIB)
+
+$(STATLIB):
+	cargo build --target=$(TARGET) --lib --release --manifest-path=./rust/Cargo.toml
+
+C_clean:
+	rm -Rf $(SHLIB) $(STATLIB) $(OBJECTS)
+
+clean:
+	rm -Rf $(SHLIB) $(STATLIB) $(OBJECTS) rust/target

src/entrypoint.c

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+// We need to forward routine registration from C to Rust
+// to avoid the linker removing the static library.
+
+void R_init_libkrigingtemplate_extendr(void *dll);
+
+void R_init_libkrigingtemplate(void *dll) {
+    R_init_libkrigingtemplate_extendr(dll);
+}

src/rust/Cargo.toml

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+[package]
+name = 'libkrigingtemplate'
+version = '0.1.0'
+edition = '2018'
+
+[lib]
+crate-type = [ 'staticlib' ]
+
+[dependencies]
+extendr-api = '*'

src/rust/src/lib.rs

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+use extendr_api::prelude::*;
+
+/// Return string `"Hello world!"` to R.
+/// @export
+#[extendr]
+fn hello_world() -> &'static str {
+    "Hello world!"
+}
+
+// Macro to generate exports.
+// This ensures exported functions are registered with R.
+// See corresponding C code in `entrypoint.c`.
+extendr_module! {
+    mod libkrigingtemplate;
+    fn hello_world;
+}