jsonschema: pre-compile regexps

This is the first CL that deals with the process of preparing a schema for validation.

Perform some basic checks on the scheme. Along the way, compile regexps and store
them in the schema for use during validation.

Change-Id: I0e5e6ce28656dcecb6d2d4b2fdc98998fa05b6f1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/669696
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

internal/jsonschema/schema.go

@@ -13,6 +13,7 @@ import (
 	"fmt"
 	"iter"
 	"math"
+	"regexp"
 )
 
 // A Schema is a JSON schema object.
@@ -106,6 +107,10 @@ type Schema struct {
 	Then             *Schema            `json:"then,omitempty"`
 	Else             *Schema            `json:"else,omitempty"`
 	DependentSchemas map[string]*Schema `json:"dependentSchemas,omitempty"`
+
+	// computed fields
+	pattern           *regexp.Regexp
+	patternProperties map[*regexp.Regexp]*Schema
 }
 
 // String returns a short description of the schema.