Add basic windows install script.

Adds a powershell install script for windows users that are at least
running powershell 5. It is almost a direct port of install.sh.
This would be used to automatically download the script much like
curl | bash, but with:

`Invoke-WebRequest https://urltoscript.com | Select-Object
-ExpandProperty Content | Out-File $env:TMP/install_rustlings.ps1;
Unblock-File $env:TMP/install_rustlings.ps1; Invoke-Expression
$env:TMP/install_rustlings.ps1`

Author: gdoenlen

install.ps1

@@ -0,0 +1,92 @@
+#!/usr/bin/env pwsh
+
+#Requires -Version 5
+param($path = "$pwd/rustlings")
+
+if((Get-ExecutionPolicy) -gt 'RemoteSigned' -or (Get-ExecutionPolicy) -eq 'ByPass') {
+    Write-Output "PowerShell requires an execution policy of 'RemoteSigned' to run this script."
+    Write-Output "To make this change please run:"
+    Write-Output "'Set-ExecutionPolicy RemoteSigned -scope CurrentUser'"
+    exit 1 
+}
+
+Write-Host "Let's get you set up with Rustlings!"
+
+Write-Host "Checking requirements..."
+if (Get-Command git -ErrorAction SilentlyContinue) {
+    Write-Host "SUCCESS: Git is installed"
+} else {
+    Write-Host "WARNING: Git does not seem to be installed."
+    Write-Host "Please download Git using your package manager or over https://git-scm.com/!"
+    exit 1
+}
+
+if (Get-Command rustc -ErrorAction SilentlyContinue) {
+    Write-Host "SUCCESS:  Rust is installed"
+} else {
+    Write-Host "WARNING: Rust does not seem to be installed."
+    Write-Host "Please download Rust using https://rustup.rs!"
+    exit 1
+}
+
+if (Get-Command cargo -ErrorAction SilentlyContinue) {
+    Write-Host "SUCCESS: Cargo is installed"
+} else {
+    Write-Host "WARNING: Cargo does not seem to be installed."
+    Write-Host "Please download Rust and Cargo using https://rustup.rs!"
+    exit 1
+}
+
+# Function that compares two versions strings v1 and v2 given in arguments (e.g 1.31 and 1.33.0).
+# Returns 1 if v1 > v2, 0 if v1 == v2, 2 if v1 < v2.
+function vercomp($v1, $v2) {
+    if ($v1 -eq $v2) {
+       return 0
+    } 
+
+    $v1 = $v1.Replace(".", "0")
+    $v2 = $v2.Replace(".", "0")
+    if ($v1.Length -gt $v2.Length) {
+        $v2 = $v2.PadRight($v1.Length, "0")
+    } else {
+        $v1 = $v1.PadRight($v2.Length, "0")
+    }
+
+    if ($v1 -gt $v2) {
+        return 1
+    } else {
+        return 2
+    }
+}
+
+$rustVersion = $(rustc --version).Split(" ")[1]
+$minRustVersion = "1.31"
+if ((vercomp $rustVersion $minRustVersion) -eq 2) {
+    Write-Host "WARNING: Rust version is too old: $rustVersion - needs at least $minRustVersion"
+    Write-Host "Please update Rust with 'rustup update'"
+    exit 1
+} else {
+    Write-Host "SUCCESS: Rust is up to date"
+}
+
+Write-Host "Cloning Rustlings at $path"
+git clone -q https://github.com/rust-lang/rustlings $path
+if (!($LASTEXITCODE -eq 0)) {
+    exit 1
+}
+
+# UseBasicParsing is deprecated, pwsh 6 or above will automatically use it,
+# but anyone running pwsh 5 will have to pass the argument.
+$version = Invoke-WebRequest -UseBasicParsing https://api.github.com/repos/rust-lang/rustlings/releases/latest `
+    | ConvertFrom-Json | Select-Object -ExpandProperty tag_name
+Write-Host "Checking out version $version..."
+Set-Location $path
+git checkout -q tags/$version
+
+Write-Host "Installing the 'rustlings' executable..."
+cargo install --force --path .
+if (!(Get-Command rustlings -ErrorAction SilentlyContinue)) {
+    Write-Host "WARNING: Please check that you have '~/.cargo/bin' in your PATH environment variable!"
+}
+
+Write-Host "All done! Run 'rustlings' to get started."