rand: add full precision f32 and f64 random functions; fix f32/f64 multipliers (#16875)

vlib/rand/README.md

@@ -11,7 +11,7 @@ import rand
 ...
 
 // Optionally seed the default generator
-rand.seed([u32(3110), 50714])
+rand.seed([u32(3223878742), 1732001562])
 
 ...
 
@@ -61,21 +61,40 @@ Otherwise, there is feature parity between the generator functions and the top-l
 A PRNG is a Pseudo Random Number Generator.
 Computers cannot generate truly random numbers without an external source of noise or entropy.
 We can use algorithms to generate sequences of seemingly random numbers,
-but their outputs will always be deterministic.
-This is often useful for simulations that need the same starting seed.
+but their outputs will always be deterministic, according to the seed values.
+
+This is often useful for simulations that need the same starting seeds.
+You may be debugging a program and want to restart it with the same
+seeds, or you want to verify a working program is still
+operating identically after compiler or operating system updates.
 
 If you need truly random numbers that are going to be used for cryptography,
 use the `crypto.rand` module.
 
 # Seeding Functions
 
-All the generators are time-seeded.
+All the generators are initialized with time-based seeds.
 The helper functions publicly available in `rand.seed` module are:
 
 1. `time_seed_array()` - returns a `[]u32` that can be directly plugged into the `seed()` functions.
 2. `time_seed_32()` and `time_seed_64()` - 32-bit and 64-bit values respectively
    that are generated from the current time.
 
+When composing your own seeds, use "typical" u32 numbers, not small numbers.  This
+is especially important for PRNGs with large state, such as `mt19937`.  You can create
+random unsigned integers with openssl `rand` or with `v repl` as follows:
+
+```
+$ openssl rand -hex 4
+e3655862
+$ openssl rand -hex 4
+97c4b1db
+$ v repl
+>>> import rand
+>>> [rand.u32(),rand.u32()]
+[2132382944, 2443871665]
+```
+
 # Caveats
 
 Note that the `sys.SysRNG` struct (in the C backend) uses `C.srand()` which sets the seed globally.