Also adds `get-initial-thread`, since threa values are useful with
`compute-size[-increments]`.
Changes the compiler to inline `weak-pair?` and `ephemeron-pair?`,
since that provides better performance for `compute-size-increments`.
original commit: 57d0cc13f8e932972cba3837b4f54e9c86786091
Add optional beginning-line and beginning-column components to a
source object, so that line and column information can be recorded
independent of the file. Add `locate-source-object-source` to use
the recorded information. Add a cache for `locate-source` as enabled by
the `use-cache?` optional argument, which can avoid compilation times
that are quadratic in the number of `let-values` or `define-values`
forms.
original commit: b36fab81d5041a54ce01a422395eee79d2f930bc
