Better eval error locations for interpolation and +

[greedy]

Previously, type or coercion errors for string interpolation, path
interpolation, and plus expressions were always reported at the
beginning of the outer expression. This leads to confusing evaluation
error messages making it hard to accurately diagnose and then fix the
error.

For example, errors were reported as follows.

cannot coerce an integer to a string
1| let foo = 7; in "bar" + foo
 |                 ^

cannot add a string to an integer
1| let foo = "bar"; in 4 + foo
 |                     ^

cannot coerce an integer to a string
1| let foo = 7; in "x${foo}"
 |                  ^

This commit changes the ExprConcatStrings expression vector to store a
sequence of expressions and their expansion locations so that error
locations can be reported accurately. For interpolation, the error is
reported at the beginning of the entire ${foo}, not at the beginning
of foo because I thought this was slightly clearer. The previous
errors are now reported as:

cannot coerce an integer to a string
1| let foo = 7; in "bar" + foo
 |                         ^

cannot add a string to an integer
1| let foo = "bar"; in 4 + foo
 |                         ^

cannot coerce an integer to a string
1| let foo = 7; in "x${foo}"
 |                   ^

The error is reported at this kind of precise location even for
multi-line indented strings.

This probably helps with at least some of the cases mentioned in #561

[greedy]

An alternative implementation strategy would be pulling the position field up into the Expr superclass or adding a virtual position accessor to Expr so that ExprConcatStrings can access the nested expressions’ positions directly instead.

[greedy]

Any thoughts/feedback on this PR? I think it's a quite nice UX improvement and am keen to see it merged.