Allow textfield size to be defined in schema xml

Up to and including 4.2 codeGen would set the size of a textfield based on the varchar length and there was no way to override that setting.
We are adding a new xml tag <size>, the contents of which must correspond to the constants defined in CRM_Utils_Type (TWO, FOUR, BIG, HUGE, etc.).
Usage example (Contact.xml):
<field>
<name>nick_name</name>
<type>varchar</type>
<length>128</length>
<size>BIG</size>
... etc.
</field>
This will allow us to remove many of the template hacks which were setup to present textfields at the size users expect.