Performance of to_html improvements

the to_html function gets called a lot sometimes many thousands of times to render a page, this is because every query to the database gets passed through it, so shaving a few microseconds off this function can boost the responsiveness of vtiger considerably. It is often called to convert the same value multiple times, even if this value is an empty string or just a number, if the called strings and results are stored in a static array then it can use the array as a cache and return the prepared result very quickly, without doing in-string searches for characters we don't want. If the array is in a session superglobal it cuts down further computation, but might need memory management at some stage.