generator: expressions can generate pre-code

This patch provides a mechanism for generating
preparatory code for expressions.

This is e.g. necessary when an expression needs computations
which cannot be done in a C-Expression, like for-loops.

This will be used for sumof expressions but may be useful
elsewhere.

V2: adapt to changes in previous patches

Message-ID: <54102E9E.4060000@DemoRecorder.com>
Patch-Thread-Subject: [Xcb] xinput: make ListInputDevices work, sumof with nested expr, ...
Patch-Set: ListInputDevices
Patch-Number: libxcb 3/6
Patch-Version: V2
Signed-off-by: Christian Linhart <chris@DemoRecorder.com>