Enhance type signature characters in classfile_constants.h and improve the JVM to use type signature characters more consistently

Improve JVM handling of well known type signature characters by defining such characters in classfile_constants.h and consistently using those definitions throughout the JVM when parsing signatures.

For example add:

enum {
            JVM_SIGNATURE_ENDPACKAGE = '/',
            JVM_SIGNATURE_ENDPACKAGE_DOT = '.',
            JVM_SIGNATURE_SPECIAL = '<',
            JVM_SIGNATURE_ENDSPECIAL = '>',

In addition use more consistently already existing definitions such as:

JVM_SIGNATURE_ARRAY instead of checking explicitly for '['
JVM_SIGNATURE_CLASS instead of checking explicitly for 'L'
JVM_SIGNATURE_ENDCLASS instead of checking explicitly for ';'