Add exercise `borrow`'s subject and tests

rust/tests/borrow_test/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "borrow"
+version = "0.1.0"
+
+[[package]]
+name = "borrow_test"
+version = "0.1.0"
+dependencies = [
+ "borrow",
+]

rust/tests/borrow_test/Cargo.toml

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+[package]
+name = "borrow_test"
+version = "0.1.0"
+authors = ["Augusto <aug.ornelas@gmail.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+borrow = { path = "../../../../rust-piscine-solutions/borrow"}

rust/tests/borrow_test/src/main.rs

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+/*
+## borrow
+
+### Instructions
+
+Complete the signature and the body of the `str_len` function so it
+receives a string or a string literal and returns its length (of type usize)
+without taking ownership of the value (i.e, borrowing the value)
+
+### Example
+
+```rust
+fn main() {
+	let s = "hello";
+	let s1 = "camelCase".to_string();
+
+	println!("\tstr_len(\"{}\") = {}", s, str_len(s));
+	println!("\tstr_len(\"{}\") = {}", s1, str_len(&s1));
+}
+```
+*/
+
+use borrow::*;
+
+fn main() {
+	let s = "hello";
+	let s1 = "camelCase".to_string();
+
+	println!("\tstr_len(\"{}\") = {}", s, str_len(s));
+	println!("\tstr_len(\"{}\") = {}", s1, str_len(&s1));
+}
+
+#[test]
+// maybe not the best way to make the test, but I wanted to use
+// lifetimes
+fn str_len_test() {
+	struct TstLit<'a> {
+		str: &'a str,
+		l: usize,
+	}
+
+	struct TstString {
+		str: String,
+		l: usize,
+	}
+
+	let tsts = vec![
+		TstLit { str: "hello", l: 5 },
+		TstLit { str: "how", l: 3 },
+		TstLit {
+			str: "are you",
+			l: 7,
+		},
+		TstLit {
+			str: "change",
+			l: 6,
+		},
+	];
+	let o_tsts = vec![
+		TstString {
+			str: "hello".to_string(),
+			l: 5,
+		},
+		TstString {
+			str: "how".to_string(),
+			l: 3,
+		},
+		TstString {
+			str: "are you".to_string(),
+			l: 7,
+		},
+		TstString {
+			str: "change".to_string(),
+			l: 6,
+		},
+	];
+
+	for t in tsts.iter() {
+		assert_eq!(t.l, str_len(t.str));
+	}
+
+	for t in o_tsts.iter() {
+		assert_eq!(t.l, str_len(&t.str));
+	}
+}

subjects/borrow/README.md

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+## borrow
+
+### Instructions
+
+Complete the signature and the body of the `str_len` that receives a string or a string literal and returns its length without taking ownership of the value (i.e, borrowing the value)
+
+### Expected Function
+
+```rust
+pub fn str_len(s: ) -> usize {
+}
+```
+
+### Usage
+
+Here is a possible program to test your function :
+
+```rust
+fn main() {
+	let s = "hello";
+	let s1 = "camelCase".to_string();
+
+	println!("\tstr_len(\"{}\") = {}", s, str_len(s));
+	println!("\tstr_len(\"{}\") = {}", s1, str_len(&s1));
+}
+```
+
+And its output:
+
+```rust
+student@ubuntu:~/[[ROOT]]/test$ cargo run
+str_len("hello") = 5
+str_len("camelCase") = 9
+student@ubuntu:~/[[ROOT]]/test$
+```