Add shift_arithmetic_left for signed integers

[remix7531]

Add shift_arithmetic_left to FStar.Int and FStar.Int{8,16,32,64,128}. This performs a left shift on the two's complement representation without requiring the input to be non-negative.

The existing shift_left requires 0 <= a, matching C semantics where left-shifting a negative signed integer is undefined behavior. shift_arithmetic_left accepts any signed integer, matching the semantics of << on signed integers in Rust and C++20+.

Changes:

• FStar.Int.fsti: definition + two bit-level lemmas
• FStar.Int.fst: trivial proofs for the lemmas
• .scripts/FStar.IntN.fstip: val + infix operator (<<<^)
• .scripts/FStar.IntN.fstp: implementation
• Regenerated FStar.Int{8,16,32,64,128}.{fsti,fst} via mk_int.sh

This commit was created with the assistance of Claude (Anthropic).

[remix7531]

The name shift_arithmetic_left was chosen for symmetry with the existing shift_arithmetic_right. However, the term is a misnomer. Arithmetic and logical shifts only differ for right shifts (sign-extending vs zero-filling). For left shifts, both operations are identical at the bit level. The only difference here is the relaxed precondition (negative inputs are allowed).

Alternatives considered, open to suggestions on naming:

• shift_left_twos_complement: more precise, but verbose
• shift_left_unrestricted: describes the relaxed precondition, but doesn't convey semantics

Relaxing the precondition on the existing shift_left, would break existing C-orientation as that would be undefined behavior.