Add expr_contains_float() and symrec_get_equ() functions. These are the first

Add expr_contains_float() and symrec_get_equ() functions.  These are the first
of many functions like this which will be needed for advanced validity checking
in the finalization of the parser stage.
Fix yacc grammar to not use the (incorrect) expr_no_fltstr, and change memexp
to memexpr and its handling to prepare for memory expressions.

svn path=/trunk/yasm/; revision=278