Segregated symbols into 5 separate namespaces

The different namespaces are:

  Function      Value expression functions, which receive a "context"
  Option        Command-line options
  Precommand    Commands which are invoked before reading the journal
  Command       Commands which are invoked after reading the journal
  Directive     Directives that occur at column 0 in a data file

This greatly eases the ability for Python uses to add intercept hooks to
change how the basic Ledger module functions.  An example of what should
be possible soon:

  import ledger

  def my_foo_handler(value):
      print "--foo received:", value

  ledger.add_handler(ledger.Option, "foo=", my_foo_handler)

1 files changed, 5 insertions, 1 deletions

diff --git a/src/account.cc b/src/account.cc
index 23761049..2a75a6ac 100644
--- a/src/account.cc
+++ b/src/account.cc
@@ -241,8 +241,12 @@ namespace {
   }
 }
 
-expr_t::ptr_op_t account_t::lookup(const string& name)
+expr_t::ptr_op_t account_t::lookup(const symbol_t::kind_t kind,
+				   const string& name)
 {
+  if (kind != symbol_t::FUNCTION)
+    return NULL;
+
   switch (name[0]) {
   case 'a':
     if (name == "amount")