Migrate all pathEnd.go message construction functions to msgs.go and use Chain as the constructing type

[colin-axner]

Currently message construction methods are being created by a PathEnd. This is unnecessary layering of information. We can move all these method construction to a msgs.go file. The PathEnd embedded in a chain is simply a pointer and not a list of pointers so trying to use a chain to house multiple path ends won't work and would require significant refactor. It'd be easier to simply create multiple chains (that refer to the same actual chain) but with different path end values. Moving the message construction functions to a chain constructor is required to fix #367 and would reduce potential bugs by enforcing that all message signers are the signer of the chain creating the message, rather than manually passing this argument in for every message construction call

[colin-axner]

see #369 for reference