Syntax differences from thtk in seqmap syntax

[ExpHP]

thtk eclmaps use _ to denote an empty strings. This is part of the seqmap syntax itself, documented at thpatch/thtk#63.

I didn't realize this, and used _ to mean something else. (the dummy args in STD)

So in thtk where you have

!ins_signatures
0 _
1 S
2 So

in truth it is

!ins_signatures
0
1 S
2 So

There's some other issues here. That PR further goes to explicitly document what should happen if a section header appears twice, or if an individual entry gets redefined. I'm fairly certain that both of these will make truth report an error.

While truth obviously needs to extend signature syntax to support more fine-grained types, it is unfortunate that it diverges from thtk on such basic properties of the text file itself...

[ExpHP]

This is a "tentative wontfix" because I'd kind of rather just replace mapfile syntax, and thanks to differences in the signature syntax itself, people who have signatures in their thtk mapfiles would need to update them to work with truth even without these problems. It'd be nice to have a much richer form of signature definitions than is even possible with seqmap syntax. Something like C-style declaration syntax

[ExpHP]

Well Egor made a ninja change and apparently _ in thtk is now deprecated, so that's convenient.

There do remain some problems with redefinitions within a file. The plan is that truth should treat these identical to the way it already treats the same thing defined in two different files.

Basically just need to change some of the HashMaps used internally for Eclmap to simple Vecs for the most part.