docs(examples): add reusable library modules

examples/lib/mathx.lumen

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+// Numeric helpers built on the math builtins.
+
+fn factorial(n) {
+    var product = 1
+    var i = 2
+    while (i <= n) {
+        product = product * i
+        i = i + 1
+    }
+    return product
+}
+
+fn gcd(a, b) {
+    var x = abs(a)
+    var y = abs(b)
+    while (y != 0) {
+        let remainder = x % y
+        x = y
+        y = remainder
+    }
+    return x
+}
+
+fn clamp(value, low, high) {
+    return max(low, min(value, high))
+}
+
+fn isPrime(n) {
+    if (n < 2) {
+        return false
+    }
+    var divisor = 2
+    while (divisor * divisor <= n) {
+        if (n % divisor == 0) {
+            return false
+        }
+        divisor = divisor + 1
+    }
+    return true
+}

examples/lib/text.lumen

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+// Text helpers built on the standard string methods.
+
+fn capitalize(text) {
+    if (text.len() == 0) {
+        return text
+    }
+    let head = text.substring(0, 1).upper()
+    let rest = text.substring(1, text.len())
+    return head + rest
+}
+
+fn repeat(text, times) {
+    var joined = ""
+    var i = 0
+    while (i < times) {
+        joined = joined + text
+        i = i + 1
+    }
+    return joined
+}
+
+fn padLeft(text, width, filler) {
+    var padded = text
+    while (padded.len() < width) {
+        padded = filler + padded
+    }
+    return padded
+}
+
+fn trimAll(parts) {
+    return parts.map(fn(part) { return part.trim() })
+}