Introduced struct pike_type in preparation for new implementation of type type.

Added copy_type() & free_type() for the same reason.
f_cast() now expects a value of type type as the first argument.

Rev: src/builtin_functions.c:1.342
Rev: src/constants.c:1.25
Rev: src/constants.h:1.16
Rev: src/docode.c:1.104
Rev: src/global.h:1.57
Rev: src/interpret.h:1.77
Rev: src/interpret_functions.h:1.45
Rev: src/language.yacc:1.224
Rev: src/las.c:1.234
Rev: src/las.h:1.47
Rev: src/opcodes.c:1.99
Rev: src/opcodes.h:1.12
Rev: src/pike_types.c:1.147
Rev: src/pike_types.h:1.48
Rev: src/program.c:1.295
Rev: src/program.h:1.116
Rev: src/svalue.h:1.76
Rev: src/treeopt.in:1.56