Introduce typedef for declaration PADOFFSET => typed-OP mapping

DRY and typemaps might not cooperate with fancy C++ types including
commas in their template arguments.
AST: vec, scalar assignment
Avoid emitting dead pushmarks as unknown OP trees
Distinguish between IV/UV/NV constants in AST

Strings are still TODO. Also, can there be any other constants (in the
OP_CONST sense) in Perl?
ASTs for Numeric vs. String constants

Splitting constant (now an abstract base) into NumericConstant and
StringConstant. Could argue that there should be IntegerConstant
UnsignedIntegerConstant and FloatConstant or similar, but that seemed
overkill right now.
AST: printf/sprintf; skip pushmarks (FIXME?)
AST: join
AST: ||= &&= //=
AST: a number of IO functions: read, getc, print, say
AST: A bunch of time related function + alarm + sleep
More AST related things to think about