basic_istream const

[malachib]

Although std isn't friendly to const char for streams, we want to be. Task items:

1. Make estd::layer2::basic_istringstream and friends const char friendly
2. This includes accepting an estd::layer2::const_string initializer

Bugfix: >> thinks a char is an integer, at least during ESP-IDF v5.3.1 compilation

Optional but potentially useful tasks:

1. Silently auto-promote to const char inside
2. estd::hex during >>