Replace all const evaluation with miri
* error reporting in constants prints a stacktrace through all called const fns
* Trivial constant propagation and folding in MIR (always active, irrelevant of the optimization level)
* can now use floating constants in patterns (previously only floating point literals were allowed)
* the future compat lint is still produced for both cases
* can index into constant arrays during const eval (previously feature gated)
* can create a constant union value with field `a` and read from field `b`
* can dereference references into constants
* can create references inside constants (`const X: &u32 = &22`)
* Tuple struct constructors can be used in constants
* regression in const eval errors spans (some of these need improvements in mir debug info)
* can cast floats to ints and vice versa (in constants, and even nan/inf constants)
* Mir dump prints false/true instead of 0u8/1u8
* `1i8 >> [8][0]` does not lint about exceeding bitshifts anymore.
* Needs const propagation across projections
* `foo[I]` produces a const eval lint if `foo: [T; N]` and `N < I`
* Essentially all builtin panics produce lints if they can be statically proven to trigger at runtime. This is on a best effort basis, so there might be some complex cases that don't trigger. (The runtime panic stays there, irrelevant of whether the lint is produced or not)
* can use `union`s to implement `transmute` for `Copy` types in constants without a feature gate. With all the greatness and nasal demons that come with this.
* can convert integers to `&'static T` in constants (useful for embedded)
fixes#34997 (stack overflow with many constants)
fixes#25574 (deref byte strings in patterns)
fixes#27918 (broken mir ICE)
fixes#46114 (ICE on struct constructors in patterns)
fixes#37448 (`SomeStruct { foo } as SomeStruct`)
fixes#43754 (`return` in const fn)
fixes#41898 (tuple struct constructors)
fixes#31364 (infinite recursion with const fn, fixed by miri's recursion limit)
closes#29947 (const indexing stabilization)
fixes#45044 (pattern matching repeat expressions)
fixes#47971 (ICE on const fn + references)
fixes#48081 (ICE on cyclic assoc const error)
fixes#48746 (nonhelpful error message with unions)
r? @eddyb
even though 1k loc are added in tests, this PR reduces the loc in this repository by 700
