parser option that skips the executable parts of a code

[rupertford]

In some situations we only need to parse the declarations of a file. There should be a way to specify this and take advantage of any potential performance improvement.

An example is where we are looking for a declaration in another module.

Another example is for the FORD document generator which does not require information about executable statements.

[rupertford]

Created branch 338_skip_exec_stmts

[rupertford]

I have a working version and am seeing performance improvements of around 20%, so nothing earth shattering, but I guess it all helps.

[rupertford]

Making the blockbase match for subroutine strict improves performance quite a bit too. We're down from around 50s to 30s on my laptop using the benchmark in #329 (although the performance from run to run seems to vary quite a bit).