Improvements to bytes

[s-hadinger]

Improvements to bytes() handling already used for some time in Tasmota:

• fixed conversion from string to bytes when the string contained NULL characters
• file.write() now supports bytes as argument in addition to string.
• file.readbytes() directly returns raw bytes() object, and internally avoid double allocation of memory. The bytes object is directly allocated with the right size and filled in-place
• added be_isbytes() to easily test if an argument is derived from the bytes class

New methods for bytes:

• getbits and setbits to change a bit-field within bytes. The code is in Berry and solidified
• added set() function mirroring the get() function
• added signed alternative geti() to get which remains unsigned. Signed/unsigned only make a difference for values smaller than the int internal size.