Improve HTTPHeaders description performance

Motivation:

As outlined in this [issue](apple#2930) by @weissi, the current performance of calling `description`
on `HTTPHeaders` is undermined by the dynamism of Array.

Modifications:

The proposed solution replaces the implementation of `description` to iterate over the items and print them out manually. This provides a faster solution since we bypass the cost of calling into `description` of an Array.

Result:

A more performant implementation of `HTTPHeaders` description.

Sources/NIOHTTP1/HTTPTypes.swift

@@ -313,7 +313,10 @@ public struct HTTPHeaders: CustomStringConvertible, ExpressibleByDictionaryLiter
     internal var keepAliveState: KeepAliveState = .unknown
 
     public var description: String {
-        self.headers.description
+        self.headers.lazy.map {
+            "\($0.0): \($0.1)"
+        }
+        .joined(separator: "; ")
     }
 
     internal var names: [String] {

Tests/NIOHTTP1Tests/HTTPHeadersTest.swift

@@ -400,4 +400,26 @@ class HTTPHeadersTest: XCTestCase {
         headers.reserveCapacity(4)
         XCTAssertEqual(headers.capacity, 4)
     }
+
+    func testHTTPHeadersDescription() {
+        let originalHeaders = [
+            ("User-Agent", "1"),
+            ("host", "2"),
+            ("X-SOMETHING", "3"),
+            ("SET-COOKIE", "foo=bar"),
+            ("Set-Cookie", "buz=cux"),
+        ]
+
+        let headers = HTTPHeaders(originalHeaders)
+
+        let expectedOutput = """
+            User-Agent: 1; \
+            host: 2; \
+            X-SOMETHING: 3; \
+            SET-COOKIE: foo=bar; \
+            Set-Cookie: buz=cux
+            """
+
+        XCTAssertEqual(expectedOutput, headers.description)
+    }
 }