One image request type instead of five (closes #307)

The new package internal/imagerequest holds the request type, its two
parts Source and Transform, the Format and Fit types, the dimension and
fit checks, the /v1/image/ path parsing and the variant key; it imports
no other package of pixa. imgcache.ParsedURL, imgcache.ImageRequest,
signature.Request, imageprocessor.Request, the second Size, format and
fit types and the conversions between them are gone. The image
processor takes a Transform only. encurl keeps a private struct with the
token's short CBOR field names. VariantKey moves with the function that
makes it. Tests follow the moved and renamed code; the tests of the
removed conversions go with them.

Model: opus-5-5

internal/imagerequest/variantkey.go

@@ -0,0 +1,39 @@
+package imagerequest
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"strings"
+)
+
+// VariantKey is a SHA256 hash identifying a specific image variant (hex-encoded).
+type VariantKey string
+
+// CacheKey generates a unique key for a request variant.
+// Format: sha256(host:path:query:width:height:format:quality:fit_mode)
+// The path and query go through escapePercentAndColon, so that a ":" in
+// either cannot be taken for the one between them.
+func CacheKey(req *Request) VariantKey {
+	data := fmt.Sprintf("%s:%s:%s:%d:%d:%s:%d:%s",
+		req.Source.Host,
+		escapePercentAndColon(req.Source.Path),
+		escapePercentAndColon(req.Source.Query),
+		req.Transform.Size.Width,
+		req.Transform.Size.Height,
+		req.Transform.Format,
+		req.Transform.Quality,
+		req.Transform.FitMode,
+	)
+	h := sha256.Sum256([]byte(data))
+
+	return VariantKey(hex.EncodeToString(h[:]))
+}
+
+// escapePercentAndColon returns s with each "%" written as "%25" and each
+// ":" as "%3A", so that the result holds no ":".
+func escapePercentAndColon(s string) string {
+	s = strings.ReplaceAll(s, "%", "%25")
+
+	return strings.ReplaceAll(s, ":", "%3A")
+}