Insert prelude at start of source file by default

[JonathanFoo0523]

Dredd previously inserted the prelude before the first function to avoid
placing it before feature test macros or headers that must precede other
headers. However, this heuristic often fails in files with class methods
using the override modifier, which is common. We now insert the
prelude at the start of the file. In rare cases where the prelude must
follow feature test macros or specific headers, manual insertion of
void __dredd_prelude_start(); will be required.

Fixes #333 .

[afd]

LGTM - please attend to the minor change and then feel free to merge.

[afd]

@JamesLee-Jones FYI - we are reverting to putting the prelude at the start of the file, but see a recent change that introduce a new __dredd_prelude_start() placeholder function that can be used to specify a different location for the prelude (useful if feature test macros are present).

[afd]

Due to #337 this will need a rebase and force-push, and the expectation files will need to be regenerated.