Rescan substituted macro arguments so deferred calls expand like gcc - #713

A function-like macro name that only becomes a call after its neighbours were substituted was never expanded. This fixes #226.

#define EMPTY()
#define EVAL(...) __VA_ARGS__
#define HELLO() "Hello World"
EVAL ( HELLO EMPTY() () )

simplecpp produced HELLO () where gcc and clang produce "Hello World". The same gap broke the DEFER/OBSTRUCT/EVAL idioms (cloak.h style), so iteration macros like REPEAT and WHILE never terminated correctly.

• Rescan substituted arguments. After an argument is pre-expanded and substituted into the replacement list, the result is rescanned together with the tokens that follow it, so a name followed by a ( that came from a different argument or from the replacement list is expanded as a call.
• Model the rescan cursor. A new Token::skippedByRescan flag records that the rescan already passed a function-like name without finding (. A ( produced later by another expansion does not turn it into a call, until the token is substituted into a new replacement list and rescanned again. This keeps EVAL(HELLO DEFER(EMPTY)() ()) at HELLO (), matching gcc.
• One trailing-call loop. expandTrailingCalls is the only place that expands a trailing macro name, with a parameter-aware lookahead for arguments that come from a parameter. Macro::expand marks its own trailing token when a call expanded to nothing, and the argument pre-expansion clears the flag at the substitution point. The flag sits next to whitespaceahead, so sizeof(Token) is unchanged.
• Common path unchanged. Arguments without any macro name skip the extra rescan.