field10x26: change type of uX to uint32_t

Change the type of uX in `secp256k1_fe_mul_inner` and `secp256k1_fe_sqr_inner` to `uint32_t`, and cast only to `uint64_t` when necessary. 

On X86 the difference was in the noise bound, but for ARM this reduces code and required stack size, and increases performance significantly. Apparantly, GCC is less smart there and sees this as a hint that `uint32_t`*`uint32_t`→`uint64_t` instructions can be used.

Before ([asm](http://www.hastebin.com/uhiburiron.armasm)):

test | min | avg | max
-----|-------|------|-------
inv | 384.321us | 384.537us | 385.125us
recover | 2845.161us | 2845.341us | 2845.426us
sign | 1324.141us | 1324.300us | 1324.453us
verify | 2672.787us | 2673.188us | 2673.498us

After ([asm](http://www.hastebin.com/afekihumoz.armasm)):

test | min | avg | max
-----|-------|------|-------
inv | 384.400us | 384.614us | 385.357us
recover | 2549.572us | 2550.015us | 2552.387us
sign | 1223.006us | 1223.169us | 1223.313us
verify | 2410.431us | 2410.598us | 2410.885us

Eg, a 10% speedup for verification.

I still intend to do a NEON implementation, but this picks a bit of low-hanging fruit that I expect to be significant on other embedded 32-bit architectures as well.