Better line number reporting for monadic binds

[langston-barrett]

Progress on #415

Example improved message:

Abort due to false assumption:
  All overrides failed during structural matching:
  *  Name: EVP_DigestUpdate
     Location: /build/fizz-hkdf/digest.saw:211:3
     Argument types: 
       %struct.env_md_ctx_st*
       i8*
       i64
     Return type: i32
     at /build/fizz-hkdf/sha256.saw:56:3
     error when loading through pointer that appeared in the override's points-to precondition(s):
     Precondition:
       Pointer: symbolic integer:  width = 64
       Pointee: symbolic integer:  width = 32
       Assertion made at: /build/fizz-hkdf/sha256.saw:56:3
     Failure reason: 
       When reading through pointer bvSum c@2346:bv 0x6c:[64]
       Tried to read something of size: 4
       And type: i32
       Found 0 possibly matching allocation(s):

Note that the precondition has a different source location than the override definition site (as opposed to e.g. the message shown in #475). Yay!

[atomb]

Definitely an improvement. And the code all seems straightforward and well-organized.