Better handling of BigWord hex encodings
Fix `hx ec-int-mod[pn]` for numbers larger than `p` or `n`

Internally we no longer uses the BigWord constructor.
Add more tests about ec-int-mod[pn] and ec-multiply.
Add more tests
[refactor] move show01,read01 to Util
[refactor] move some functions to Utils
Util: add decodeHexBytes
Electrum deterministic wallet support