Handle precision statements properly in the parse tree

[afd]

As a shortcut from the early days of the project, the PrecisionDeclaration class has a String field that captures the text from the input file associated with a precision statement.

This should be changed so that the class has fields:

• TypeQualifier precisionQualifier;
• Type type;

The precisionQualifier field should only be allowed to be HIGHP, MEDIUMP or LOWP.

The type field should only allowed to be BasicType.INT, BasicType.FLOAT, or an instance of SamplerType.