C-API missing features

[mratsim]

• Scalar Mul G1 was supposed to be in C & Rust elliptic bindings: batchAffine, scalarMul, serial MSM and 32-bit Rust bindings #332 but skipped due to the header generator macro skipping the when ECP.G == G1 here:
  

  constantine/bindings/c_curve_decls.nim

  Lines 359 to 369 in 89d32a9

  	func `ctt _ ECP _ batch_affine`(dst: ptr UncheckedArray[ECP_Aff], src: ptr UncheckedArray[ECP], n: csize_t) =
  	dst.batchAffine(src, cast[int](n))
  	when ECP.G == G1:
  	# Workaround gensym issue in templates like mulCheckSparse
  	# for {.noInit.} temporaries and probably generic sandwich
  	func `ctt _ ECP _ scalar_mul_big_coef`(
  	P: var ECP, scalar: ScalarBig) =
  	P.scalarMul(scalar)

  
  

  constantine/bindings/macro_curves_bindings.nim

  Lines 21 to 28 in 89d32a9

  	macro collectBindings*(cBindingsStr: untyped, body: typed): untyped =
  	## Collect function definitions from a generator template
  	var cBindings: string
  	for generator in body:
  	generator.expectKind(nnkStmtList)
  	for fnDef in generator:
  	if fnDef.kind notin {nnkProcDef, nnkFuncDef}:
  	continue

  
  The when branch was not a function so ignored
• Scalar Mul G2: This also fixes C/Rust library: expose G2 scalar multiplication #333
• Conversion between Fr, Big, serialized octet-strings