Add explicit generic conversion operators (#56)

The new `to` operators that can be used to constuct Stint types from
regular integer types. These are intented for usage in generic code
such as rlp.nim

Also fixes a bug preventing construction of Stint values from unsigned
integers (because they lack an unary `-` operator).

stint/io.nim

@@ -56,14 +56,23 @@ func stint*[T: SomeInteger](n: T, bits: static[int]): StInt[bits] {.inline.}=
 
   when result.data is IntImpl:
     static_check_size(T, bits)
-    if n < 0:
-      assign_least_significant_words(result, -n)
-      result = -result
+    when T is SomeSignedInt:
+      if n < 0:
+        assign_least_significant_words(result, -n)
+        result = -result
+      else:
+        assign_least_significant_words(result, n)
     else:
       assign_least_significant_words(result, n)
   else:
     result.data = (type result.data)(n)
 
+func to*(x: SomeInteger, T: typedesc[Stint]): T =
+  stint(x, result.bits)
+
+func to*(x: SomeUnsignedInt, T: typedesc[StUint]): T =
+  stuint(x, result.bits)
+
 func toInt*(num: Stint or StUint): int {.inline.}=
   # Returns as int. Result is modulo 2^(sizeof(int)
   num.data.least_significant_word.int

tests/test_io.nim

@@ -113,6 +113,17 @@ suite "Testing conversion functions: Hex, Bytes, Endianness using secp256k1 curv
     SECPK1_N = "115792089237316195423570985008687907852837564279074904382605163141518161494337".u256
     SECPK1_N_BYTES = [byte(255), 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 254, 186, 174, 220, 230, 175, 72, 160, 59, 191, 210, 94, 140, 208, 54, 65, 65]
 
+  test "explicit conversions from basic types":
+    type
+      UInt256 = Stuint[256]
+      Int128 = Stint[128]
+
+    let x = 10.uint16
+
+    check:
+      x.to(UInt256).bits == 256
+      x.to(Int128).bits == 128
+
   test "hex -> uint256":
     check: SECPK1_N_HEX.parse(Stuint[256], base = 16) == SECPK1_N