Base32 followups, Features serialization

[optout21]

Some followups to Base32/Bech32 changes #3270

Bolt11InvoiceFeatures are serialized to 5-bit values internally, but in a way that is not compatible with Bech32. The grouping of bits is done in reverse (both byte order and bit order). The current native implementation could be switch to be based on rust-bech32 (increased reuse), and iterative approach (less allocations). See:
#3270 (comment)

Change fn from_base32(b32: &[Fe32]) to take iterator instead of slice; see #3270 (comment)

Also: (minor) #3270 (comment)

[TheBlueMatt]

Also, after we bump our MSRV we can drop all the Boxes we added in serialization.

[optout21]

In rust-bech32, option with no-trimming 5-bit-to-8-bit conversion is on its way. When it's available (in a new release), the pre-padding in LDK can be removed -- in Bolt11Features serialization and in invoice hash computation.
rust-bitcoin/rust-bech32#201