fix[lang]: fix == and != bytesM folding

[trocher]

What I did

Fix [bytesN] [==|!=] [bytesN] to be folded as the operation was computed on python strings and not integer values.

How I did it

Compare lowercase strings instead of strings to be case-insensitive.

How to verify it

The test is explicit.

Before this, the following would hold.

@external
def foo() -> bool:
    a:bytes4 = 0xA1AAB33F
    b:bytes4 = 0xa1aab33f
    return a == b # returns True

@external
def bar() -> bool:
    return 0xA1AAB33F == 0xa1aab33f # returns False

Commit message

`bytesM` literals are not case-sensitive (they represent the same value
no matter if the literal is lower- or upper-case), but the folding
operation was case sensitive.

Description for the changelog

fix == and != bytes folding

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