[04/32] cpp lexer

[Nathan Sidwell]

c++ modules creates 2 new kinds of preprocessor lines
[export] module ...
[export] import ...

To all intents and purposes these are cppdirectives spelt without a 
leading '#'.  module and import are context-sensitive keywords.  Thus 
preprocessor tokenizing needs a bit of token peeking.  This is that peeking.

We have a new node flag 'NODE_MODULE', which marks whether an identifier 
is significant to this peeking.  When we see such an identifier at the 
beginning of a logical line, we need to peek further and figure out 
whether these are those keywords.

When successfully peeked, we replace the identifiers with internal-only 
tokens that the c++ parser recognizes.

-- 
Nathan Sidwell

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