lib: improve error reporting

[jow-]

Improve runtime and syntax error reporting in two ways:

Underline the failing statement

Error contexts previously marked only the single token/instruction which failed, leaving the reader to reconstruct the statement around it. The statement start/end offsets are now resolved from the offset info and underlined, keeping the caret at the failing position and dropping the now-redundant "Near here" label.

• Stacktrace entries now carry the statement extent as a statement member (start/end line and byte), useful for editors and debug clients.
• Syntax errors are reported against the innermost statement compilation was inside of.
• Fixed two flaws in the span lookup along the way: the search was abandoned one level too early, and the byte/instruction deltas of a span-start entry were accumulated alike.

Name the offending expression in call failures

When a call fails because its left-hand side isn't a function, the message now includes the recovered source text of the invoked expression, falling back to the generic message whenever it cannot be recovered with confidence. Expressions which do not fit are cut short, marked with an ellipsis.

Example

Before:

Type error: left-hand side is not a function
In fn(), line 3, byte 24:
  called from function call ([C])
  called from anonymous function ([stdin]:7:30)

 `        print("Hello world\n");`
  Near here -------------------^

After:

Type error: `print` is not a function, got null instead
In fn(), line 3, byte 24:
  called from function call ([C])
  called from anonymous function ([stdin]:7:30)

 `        print("Hello world\n");`
         ~~~~~~~~~~~~~~~~~~~~~^~