[eval2] Implement BLOCK and TAGBODY without using MACROLET.
[eval2] Clean up.
[eval2] Support null symbol macros.
[eval2] Support globally defined symbol macros.
[eval2] Fix the implementations of TAGBODY and BLOCK.
[eval2] Implement a %LOOP primitive for use in the expansion of TAGBODY.
[eval2] Fix the implementation of symbol macros.